[Article 2 of 2.] This is a summary of the alternate definitions and uses of driving logic relationships between activities in project schedules, as applied in Primavera P6 and Microsoft Project software. Driving relationships are often considered fundamental elements of the project critical path.
This winter I worked with a colleague to prepare a paper – Interpreting Logic Paths in Multi-Calendar Project Schedules – for presentation at this year’s AACE International Conference and Expo in Chicago (Covid-19) virtual world. It’s a deep dive into the Multiple Float Path calculation options in Primavera P6 scheduling software. During the technical study, I had a lot of opportunities to think about driving logic relationships. I’ve summarized the standard definitions and uses in an earlier article. This entry summarizes the alternate versions of driving logic relationships that sometimes arise.
The Importance of Driving Logic
The planning and execution of complex projects requires the project team to understand, implement, and communicate the consequences of schedule logic flow to the other stakeholders. Through schedule logic, each activity in the project has the potential to constrain or disrupt numerous other activities – and to be constrained or disrupted by them. The most obvious artifacts of logic flow are the important logic paths, like the critical path, the Longest Path (in Primavera P6), or the driving path to a key delivery milestone. Regardless of the detailed definition, each of these important paths is governed by driving logic relationships from the first activity to the last activity in the path.
Late-Dates and Bi-Directional Driving Relationships in Primavera P6
From the earlier article, a relationship is considered driving (under the standard definitions) when its successor’s early dates are constrained by the relationship, during the forward pass of the CPM calculations. That is, standard driving relationships are early-dates driving relationships. A late-dates driving relationship, in contrast, is one that constrains the late dates of the predecessor, during the backward pass. When an activity has multiple successors, then one or more of these successor relationships may be controlling the late dates, and hence the total float, of the selected activity; this is a late-dates driving relationship.
Identification of late-dates driving relationships is a key factor in P6’s Multiple Float Path (MFP) calculation. Under the total float calculation option, a relationship can be assigned to a driving “float path” only if it meets the criteria for both early-dates and late-dates driving relationships. That is, it possesses bi-directional driving logic. Since P6 does not flag or otherwise mark such relationships, the results of multiple float path calculations with the total float option can be confusing. Full understanding of the float paths may require a detailed examination of relationship and activity floats, especially when multiple calendars or constraints are involved.
For more information on MFP calculations in P6, check out these other entries in the blog:
Aside from the MFP calculation option in P6, this type of driving logic is useful mainly for prioritizing driving successors when click-tracing through the schedule network in the forward direction – perhaps for schedule validation or disruption analysis. For example, consider the selected activity (A1020, “Task”) in the P6 version of our simple project below. A glance at the late-date bars shows that only one of its five driven successors (A1060, “succ f”) is responsible for the late dates (and total float) of the selected task. The corresponding relationship possesses bi-directional driving logic and marks the forward continuation of the total-float-based driving path. In the relationship tables, the “Driving” checkboxes already indicate the relationships with early-dates driving logic. When exploring forward, most P6 users will simply click to the driven successor activity that is “Critical” or has the lowest total float value, and that will be correct much of the time. When multiple constraints and/or calendars exist, however – or when the path being explored is far from critical – then late-dates driving logic is indicated when the “Relationship Total Float” equals the total float of the predecessor activity, as highlighted in the figure.
Late-Dates and Bi-Directional Driving Relationships in BPC Logic Filter
With no built-in alternatives, BPC Logic Filter automatically identify all three types of driving relationships – early-dates, late-dates, and bi-directional – in Microsoft Project schedules. The next figure repeats the same simple example project from the earlier article, with additional bars for early and late dates (green and red) and the task paths shown earlier (orange, gold, purple.) Within the logic inspector tables, bi-directional driving relationships are highlighted (red/yellow) and shown on top. Among those relationships that are NOT bi-directional drivers, early-date drivers are shown in the same yellow as before, and late-date drivers are shown in pale red. As usual, the logic inspector’s jump buttons make for easy, logic-based navigation through the schedule.
Unlike MSP’s built-in task path bar styles, the logic inspector tables are equally effective at illustrating driving logic in backward-scheduled projects. This is demonstrated below, where the same example project has been re-configured to Schedule from: Project Finish Date. Interestingly, while the scheduled dates clearly change, the nature of driving logic relationships does not.
Resource Driving Logic Relationships in BPC Logic Filter
When resource leveling is imposed in a P6 or MSP project schedule, some tasks are delayed from their CPM-based early dates until resources become available – after completion of other tasks. In the figure below, a single resource has been assigned to the five successors of the “Task,” and the resulting overallocation of the resource has been resolved by leveling the schedule using the simplest options. As a result, the project finish milestone has been delayed by three days, and the critical path has shifted.
The leveling process creates implied driving relationships between tasks that demand the same resources. BPC Logic Filter infers these “ResDrvr” relationships. As shown below, the resulting resource-constrained driving logic paths are typically very different from those identified using CPM logic alone.
The consequences of resource driving logic are further addressed in these earlier articles:
Hierarchical (Parent-Child) Driving Logic Relationships in Microsoft Project
Unlike other project scheduling tools, MSP supports direct assignment of schedule logic (start predecessors and finish successors only) to “summary tasks.” As a consequence, it then imposes automatic logic restraints based on the relative positions of tasks within the Outline/Work breakdown structure. Thus, a summary task with a finish-to-start predecessor automatically imposes a corresponding early-start restraint on every one of its subtasks, and this restraint is inherited at each outline level all the way to the lowest-level subtask. Moreover, a summary task with a finish to start successor automatically imposes a corresponding late-finish (backward-pass) restraint on its subtasks, which is inherited all the way down the outline structure. External date constraints, manual-mode scheduled dates, and actual dates inputs for summary tasks have similar consequences.
The immediate early-start drivers for summary tasks and subtasks – whether a result of predecessor logic, outline-parent inheritance, or outline-child roll-up – can be identified by the task Inspector as shown in the next figure, and some of these are explicitly enumerated in the “driver” collections of the task. The late-date consequences remain implicit, however.
The apparent critical path for the schedule of the previous figure runs through tasks a-d and tasks e-g, including their driving FS relationships. Not shown, however, is the implicit driving FF relationship from task d to its outline parent task Sum1 (here identified in BPC’s logic inspector tool.)
The implicit driving SS relationship from task Sum2 to its outline child task e is correctly identified by the task inspector as well as BPC’s logic inspector.
Those two implicit hierarchical relationships – when combined with the explicit Sum1-to-Sum2 FS relationship – are necessary to properly calculate early and late dates and total slack, which is the source of the critical path depicted. Unfortunately, the built-in tools are not sufficient to fully trace driving logic through such hierarchical relationships, even in this simple schedule.
Neither summary-task relationships nor the consequent hierarchical (parent/child – child/parent) relationships are explicitly recognized in the generally accepted, traditional understandings of logic-based project scheduling – i.e. the critical path method (CPM) and the precedence diagramming method (PDM). Such relationships are not generally supported in other scheduling tools, either, so attempts to migrate MSP schedules containing summary logic into other tools for analysis are typically unsuccessful. It is also clear that adding even a small number of summary-task relationships to a moderately complex project schedule can potentially obfuscate the driving logic paths in the schedule, including the critical path under many circumstances, without fairly sophisticated analysis. Taking these facts together, most project scheduling professionals seem to agree that summary-task logic in MSP represents poor practice and is to be avoided.
[Article 1 of 2.] This is a summary of the standard definitions and uses of driving logic relationships between activities in project schedules, as applied in Primavera P6 and Microsoft Project software. Driving relationships are often considered fundamental elements of the project critical path.
This winter I worked with a colleague to prepare a paper – Interpreting Logic Paths in Multi-Calendar Project Schedules – for presentation at this year’s AACE International Conference and Expo in Chicago (Covid-19) virtual world. The paper reflects a deep dive into the Multiple Float Path calculation options in Primavera P6 scheduling software. During the technical study, I had a lot of opportunities to think about driving logic relationships. This entry summarizes the standard definitions and uses. I’ve summarized a couple alternate definitions and uses in another article.
The Importance of Driving Logic
The planning and execution of complex projects requires the project team to understand, implement, and communicate the consequences of schedule logic flow to the other stakeholders. Through schedule logic, each activity in the project has the potential to constrain or disrupt numerous other activities – and to be constrained or disrupted by them. The most obvious artifacts of logic flow are the important logic paths, like the critical path, the Longest Path (in Primavera P6), or the driving path to a key delivery milestone. Regardless of the detailed definition, each of these important paths is governed by driving logic relationships from the first activity to the last activity in the path.
Standard Definition and Uses of Driving Logic Relationships
A driving relationship is “A relationship between two activities in which the start or completion of the predecessor activity determines the early dates for the successor activity with multiple predecessors. See also: Free Float.” [That’s the standard definition from AACE International.] Alternately, “A driving relationship is one that controls the start or finish of a successor activity.” [That’s from the PMI publication on CPM Scheduling for Construction.]
For practical purposes, a driving relationship is a predecessor relationship that prevents a successor activity’s early start or early finish from being scheduled any earlier than it is. When an otherwise unconstrained activity has only one predecessor, then it is normally, and obviously, a driving predecessor. When an activity has multiple predecessors, then one or more of them may be driving while the others are non-driving. These distinctions answer the key questions, “Why is this activity scheduled when it is? Why can’t we do it sooner?”
Driving Logic in Primavera P6
Like its predecessors, P6 routinely illustrates driving logic relationships using solid lines on bar charts – either red or black depending on the “Critical” status of the connected activities. Non-driving relationships are depicted with the same colors, and those that are also non-critical use dotted lines. This is demonstrated in the figure below, where the non-critical activity “Task” (A1020) has two predecessors and five successors. One of the predecessor relationships and all five of the successor relationships are depicted with solid black lines and marked as driving (but not critical) in the relationship tables. The non-driving relationships – one from “pred a” to “Task” and five more from Task’s successors to the project’s finish milestone – are depicted with dotted lines. The two critical, driving relationships that connect the “CP” activity to the project’s start and finish milestones are depicted with solid red lines.
In small projects it is often easy to identify driving logic flow in printed P6 bar charts by visually tracing the solid relationship lines between activities. As project schedules become larger and more complex, however, the number of relationship lines increases to the point that visual tracing becomes impractical. Then driving relationships are primarily identified using the relevant columns of the associated relationship tables. Experienced P6 users often use the “GoTo” buttons in the relationship tables to click-trace along driving logic paths – backward and forward through complex project schedules – to review and confirm important chains of sequential logic (i.e. driving logic paths).
In general, Primavera P6 identifies driving relationships by analyzing the intervals between early dates of the linked activities, after completion of the core scheduling calculations. With a few minor exceptions, a driving relationship is identified when the Relationship Successor Free Float (RSFF) equals zero. In addition to providing a basis for the graphical and tabular depictions of driving logic flow, P6 uses these driving attributes to automatically identify the Longest Path, or the driving path to project completion.
Driving Logic in Microsoft Project
Unlike P6, Microsoft Project (MSP) does not graphically differentiate driving and non-driving relationship lines in Gantt-chart views, and the standard relationship (i.e. dependency) tables provide no driving-logic indicators. The Task Inspector pane provides the primary method for identifying driving predecessors of the currently-selected task; there is no corresponding method for identifying driven successors. The figure below depicts the same schedule as before, now in MSP format, with the Inspector pane identifying a single (driving) predecessor task, “pred b”, for the currently-selected task (“Task.”) As far as it goes, this agrees with P6.
Although not presented to users, driving relationship indicators are developed by MSP (at least since MSP 2007), with the results being stored in the PredecessorDrivers collection for each task. This collection forms the basis of the Predecessors list of the Inspector pane.
It’s also apparent that the PredecessorDrivers data are used to define the Driving Predecessors and Driven Successors bar styles that were introduced as part of the Task Path functionality in MSP 2013. This functionality is illustrated below, where the driving and non-driving predecessors of “Task” – and its driven successors – are all differentiated by bar color. Although there are clear limitations to this graphical approach, the ability to show driving and driven logic paths (if not individual driving relationships) is a major improvement for MSP users.
Unfortunately, the internal MSP calculation of driving logic attributes – and the explicit paths of driving logic that they purport to illustrate – have proven unreliable for complex schedules with other than finish-to-start relationships, out-of-sequence progress, or external links.
Standard Driving Logic in BPC Logic Filter for Microsoft Project
BPC Logic Filter (my company’s add-in for MSP) identifies driving logic by independently analyzing relationship relative floats after completion of the schedule calculations. This is a bit like the P6 approach and has proven, at least for me, more reliable than the internal MSP data when things get complicated. This figure shows the combination of a logic tracer view (with special bar styles depicting driving and near-driving logic paths) together with the task logic inspector tables. Driving relationships are highlighted yellow in the tables. Overall, this seems to combine the best parts of the corresponding P6 and MSP layouts, and the Jump buttons allow for logic-based navigation forward or backward through the schedule network.
In Project schedules, the Longest Path yields the Shortest Time. Aside from the mental gymnastics needed to digest that phrase, the concept of Longest Path – especially as implemented in current software – has deviated enough from its origins that a different term may be needed.
Critical Path as Longest Path
Authoritative definitions of the “Critical Path” in project schedules typically employ the words “longest path,” “longest chain,” or “longest sequence” of activities … (that determine the earliest completion date of the project.) In other words, the path, chain, or sequence with the greatest measured length is the Critical Path. As a rule, however, none of the associated documents are able to clearly define what constitutes the length of a logic path, nor how such length will be measured and compared in a modern project schedule. Without a clear standard for measuring the length of something, explicitly defining the Critical Path in terms of the longestanything is just sloppy in my view.
The Original Path Length
Assessing path length used to be much easier. In the early days of CPM (Critical Path Method) scheduling, any project schedule could be guaranteed to have ALL Finish-to-Start relationships, NO constraints, NO lags or leads, NO calendars, and only ONE Critical Path. Under these conditions, the length of a logic path could be clearly defined (and measured) as the sum of the durations of its member activities. Thus, the overall duration of a Project was equal to the “length” (i.e. duration) of its Critical Path, which itself was made up of the durations of its constituent activities. That result is indicated in the figure below, where the 64-day project length is determined by the durations of the 5 (highlighted) activities on the Critical Path. Adding up the activity durations along any other path in the schedule results in a corresponding path length that is less than 64-days – i.e. not the “longest” path. [The network diagram was taken from John W. Fondahl’s 1961 paper, “A Non-Computer Approach to the Critical Path Method for the Construction Industry,” which introduced what we now call the Precedence Diagramming Method. Unfortunately, Microsoft Project (MSP) has an early limit on dates, so his presumed ~1961 dates could not be matched.]
Fortunately, in such simple projects, it’s never been necessary to aggregate and compare the lengths of every logic path to select the “longest path.” The CPM backward pass calculations already identify that path by the activities with zero-Total Float/Slack, and successively “shorter” paths are identified by successively higher Total Float/Slack values. This fact has been verified in countless student exercises involving simple project schedule networks, typically concluding with the axiom that “the Critical Path equals the longest path, which equals the path of zero-Total Float/Slack.”
Float/Slack and Path-Length Difficulties
In general, modern complex project schedules have, or can be expected to have, complicating factors that make Total Float/Slack unreliable as an indicator of the Critical Path – e.g. non-Finish-to-Start relationships, various early and late constraints, multiple calendars, and even resource leveling. See this other article for details. Therefore, as noted earlier, the axiomatic definition has been shortened to “the Critical Path equals the longest path.”
Unfortunately, finding the “longest path” by arithmetically summing the activity lengths (i.e. durations) along all possible logic paths and comparing the results – not easy to begin with – has gotten more difficult. Lags, excess calendar non-working time, early constraints, and resource leveling delays all add to the true “length” of a logic path compared to the simple summation of activity durations. On the other hand, leads (negative lags), excess calendar working-time, and the use of overlapping-activity relationships (e.g. SS/FF) reduce its length. In addition, any hammocks, level-of-effort, and summary activities need to be excluded. All such factors must be accounted for if the “longest path” is to be established by the implied method of measuring and comparing path lengths in the project schedule. I don’t know of any mainstream project scheduling software that performs that kind of calculation. Alternatively, Deep Schedule AnalysisTM using the proprietary HCP (Hidden Critical Path) Method – from HCP Project Management Consulting – appears to compute and compare the lengths of all logic paths in Primavera and MSP schedules.
Longest Path as Driving Path
Contrary to summing up and comparing logic path lengths, current notions of the “longest path” are based on an approach that does not involve path “length” at all. As a key attribute, the longest path in a simple, un-progressed project schedule also happens to be the driving logic path from the start of the first project activity to the finish of the last project activity. It is a “driving logic path” because each relationship in the path is “driving”, that is it prevents its successor from being scheduled any earlier than it is. Driving relationships are typically identified during the forward-pass CPM calculations. Subsequently, the driving path to the finish of the last activity can be identified by tracing driving logic backward from that activity, terminating the trace when no driving predecessors are found or the Data Date is reached. The resulting driving path to project finish is also called the “longest path” even though its “length” has not been established. This is the “Longest Path” technique that has been applied for nearly two decades by (Oracle) Primavera and adopted more recently in other project scheduling tools.
As of today, MSP continues to define Critical tasks on the basis of Total Slack, but it provides no explicit method for identifying the “Critical Path” using a “longest path” criterion. How is the responsible MSP scheduler supposed to respond to a demand for the “critical path” when the longest path has been obscured? Here are several options:
Continue to make simple projects, avoiding all complicating factors like calendars (including resource calendars), early and late constraints, deadlines, and resource leveling. Then assume that “Total Slack = 0” correctly identifies the Critical Path.
If you are using MSP version 2013 or later,
Ensure that your project is properly scheduled with logic open-ends only present at a single start and single finish task/milestone, then select the single finish task,
Try to use the “Task Path” bar highlighter to highlight the “Driving Predecessors” of your selected finish task. In the example below, a Deadline (a non-mandatory late-finish constraint) has been applied to task Op12 in the 1961 example, and MSP has responded by applying the “Critical” flag (based on TS=0) to Op12 and its predecessors Op10 and Op2. As a result, the Critical Path is obscured. Applying the bar highlighter and selecting task Op18 (the project’s finish task) correctly identifies the driving path to project completion, i.e. the “longest path.” (For clarity, I manually added the corresponding cell highlighting in the table; the bar highlighter doesn’t do that.)
If necessary, create and apply a corresponding filter for the highlighted bars. I’ve posted a set of macros to make and apply the filter automatically in this article.
If you are using MSP version 2007 or later,
Ensure that your project is properly scheduled with logic open-ends only present at a single start and single finish task/milestone, then select the single finish task,
Try to use the Task Inspector to identify the driving predecessor of the selected task, then go to it and flag it as being part of the driving path. Repeat this until the entire driving path is marked.
If necessary, create and apply a filter and/or highlighting bar styles for the flagged tasks.
I’ve posted another set of macros to do all this (except bar highlighting) automatically in this other article.
Note: The previous two approaches both rely on MSP’s StartDriver task object to identify driving relationships. As noted in this article, however, the resulting driving logic is not reliable in the presence of tasks with multiple predecessors, non-FS predecessors, or actual progress.
Use BPC Logic Filter or some other appropriate add-in to identify the “longest path” in the schedule.
Whichever method or software is used, expressing the Longest Path using the Driving Path methodology has one key weakness: it has not been proved generally useful for analysis of near-critical paths. While the Longest Path may be known, its actual length is not readily apparent. More importantly, there is no basis for computing the lengths, and hence the relative criticality, of the 2nd, 3rd, and 4th etc. Longest Paths. Consequently, Near-Critical paths continue to be identified based on Total Float/Slack, which is still unreliable, or – in P6 – based on unit-less “Float Paths” from multiple float path analysis.
“Longest Path” and Early Constraints
As noted several times here, the methods described for identifying the “longest path” are in fact describing the “driving path to the project finish.” This distinction can raise confusion when an activity is delayed by an early constraint. Consider the case below, where an activity on the longest path (Op13) has been delayed 2 days by an early start constraint. Consequently, its sole predecessor relationship (from Op3) is no longer driving, and Op3 gains 2 days of Total Float/Slack. As shown by MSP’s “Driving Predecessor” bar highlighter, the driving logic trace is terminated (going backwards) after reaching the constrained task.
Identical results are obtained from Primavera’s (P6) Longest Path algorithm. This is neither surprising nor incorrect; the project’s completion is in fact driven by the external constraint on Op13, and its predecessor Op3 is quite properly excluded.
It’s clear therefore that the driving path to project completion and the longest path from the project start (or Data Date) to the project completion can differ when an early constraint is present. P6’s “Longest Path” algorithm automatically defaults to the driving path, not the actual longest path, and to date there have been no built-in alternatives to that behavior. As a result, some consultants suggest that P6 Longest Path analyses should be rejected when external constraints – even legitimate ones like arrival dates for Customer Furnished Equipment – are present. (A P6 add-in, Schedule Analyzer Software, does claim to provide a true Longest Path representation in the presence of early constraints.)
BPC Logic Filter – Longest Path Filter
BPC Logic Filter is a schedule analysis add-in for MSP that my company developed for internal use. The Longest Path Filter module is a pre-configured version of the software’s Task Logic Tracer. The module is specifically configured to identify the project’s longest path (as driving path) through the following actions:
Automatically find the last task (or tasks) in the project schedule.
Excluding tasks or milestones that have no logical predecessors. (E.g. completion milestones that are constrained to be scheduled at the end of the project but are not logically tied to the actual execution of the project. The resulting trace would be trivial.)
Excluding tasks or milestones that are specifically flagged to be ignored, e.g. (“hammocks”)
Trace the driving logic backwards from the last task to the beginning of the project.
Driving logic is robustly identified by direct computation and examination of relative floats. (Driving relationships have zero relative float according to the successor calendar.) The unreliable StartDriver task objects are ignored.
Neither completed nor in-progress tasks are excluded from the trace.
Either apply a filter to show only the driving logic path, or color the bars to view the driving logic path together (in-line) with the non-driving tasks. The example below is identical to the previous one, but BPC Logic Filter formats the bar chart to ignore the impacts of the applied deadline. The resulting in-line view is substantially identical to the bar chart of the original, unconstrained project schedule.
BPC Logic Filter and the (True) Longest Path
As noted earlier, an early constraint can truncate the driving path to project completion. In that case, it is debatable in my view whether the addition of non-driving, float-possessing activities into the “longest path” makes that term itself more or less useful with respect to the typical uses of the “Critical Path” in managing and controlling project performance. Nevertheless, such an addition is easily allowed in BPC Logic Filter by checking a box. The bar chart below shows the results of the Longest Path Filter on the early-constrained example schedule, as set up according to the driving-path (Primavera) standard. Results are identical to those of the built-in “Driving Predecessors” highlighter in MSP (above) and of P6.
The next chart shows the complete “longest path” for the project, including the non-driving Op3 activity.
The second chart is different because the check box for “Override if successor task is delayed by constraint” has been checked in the analysis parameters form. Checking the box causes the non-driving predecessor with the least relative float to be treated as driving, and therefore included in the Longest Path, in the event of a constraint-caused delay.
As noted earlier, the normal methods for identifying the “longest path” (i.e. the driving path) in a project schedule have not been generally adopted for analyzing near-longest paths. P6 offers multiple float path analysis, which I wrote about here. In addition, Schedule Analyzer (the P6 add-in mentioned earlier) computes what it calls the “Longest Path Value” for each activity in the schedule – this is the number of days an activity is away from being on the Longest Path (i.e. the driving path to project completion.) In the absence of demonstrated user demand, however, MSP seems unlikely to gain much beyond the Task Path bar highlighters.
BPC Logic Filter routinely computes and aggregates relative float to identify driving and near-driving logic paths in MSP project schedules. In this context, “near-driving” is quantified in terms of path relative float, i.e. days away from driving a particular end task (or days away from being driven by a particular start task.) Its “Longest Path” and “Near Longest Path” analyses are special cases where the automatically-selected end task is the last task in the project. For the Near Longest Path Filter, tasks can be shown in-line (with bar coloring) or grouped and sorted based on path relative float. The “override if successor is delayed by constraint” setting has no effect when the Near Longest Path Filter is generated. In that case, the non-driving task will be displayed according to its actual relative float. For example Op3 is shown below with a relative float of 2 days (its true value), not 0 days as shown on the earlier Longest Path Filter view.
In the development of the Critical Path Method, the “longest path” originated as one of several defining characteristics of the “Critical Path” in simple project schedules. Specifically, the “Critical Path” included the sequence of activities with the highest aggregated duration – i.e. the “longest path”. Actual computation and comparison of path lengths was not necessary since relative path lengths could be inferred directly from Total Float – a much easier calculation.
Complicating factors in modern project schedule networks tend to confuse the interpretation of Total Float, such that it is no longer a reliable surrogate for path length. As a result, the most recent, authoritative definitions of the Critical Path tend to omit references to float while retaining references to “longest path” and, typically, logical control of the project completion date. [Notably, the measurement and comparison of aggregated path durations (path lengths) has not been an explicit feature of any mainstream project scheduling tool, so the “longest-path” part of the definition cannot be definitively tested in general practice.]
Notions of “longest-path” among current schedule practitioners are heavily influenced by the deceptively-named “Longest Path” feature in Oracle/Primavera’s P6 software. Perversely, that feature DOES NOT aggregate activity durations along any logic paths. Rather, it identifies the driving/controlling logic path to the project’s early finish date.
The “Longest Path” in P6 (i.e. the Driving Path to Project Completion) and the “longest path” (i.e. the logic path with highest aggregated duration) are NOT equivalent, particularly when the “Longest Path” is constrained by an early date constraint. There is at least one P6 add-in claiming to identify the true “longest path” (and near-“longest paths”) in this case.
Microsoft Project provides several inefficient methods to identify the Driving Path to Project Completion in simple projects, but these methods are not reliable in the presence of non- Finish-to-Start relationships. There are no native MSP methods for identifying near-driving tasks nor the true “longest path” in the presence of early date constraints. BPC Logic Filter is an MSP add-in that automatically fills these gaps.
As conceived, the “longest path” criterion implied the transparent calculation and comparison of aggregated activity durations along each logic path in a project schedule. As for Total Float, however, such calculations in complex schedules have been obfuscated by complications like non- Finish-to-Start relationships, lags, and multiple calendars. Since such obfuscation makes path lengths essentially un-testable, it appears that future Critical Path definitions should omit the “longest path” criterion in favor of a simple “driving path to project completion.”
Longest Paths in Backward Scheduled Projects (MSP) [Jan’19 Edit]
As pointed out in this recent article, the Longest Path in a backward scheduled project is essentially the “driven path from the project start,” not the “driving path to project completion.”
The most recent build of BPC Logic Filter includes improved calculation of relative floats for tasks whose Resource Calendars are substantially different from the effective Task and Project Calendars. While reviewing those improvements, I compiled this summary of the three different Calendar types used in Microsoft Project (MSP) schedules – with particular attention to their use in logic-driven scheduling and Slack calculation. The summary moves from the simplest (Project Calendar only) to the most complex (combined Task and Resource calendars) case. The conclusions are based on my own (imperfect) testing in MSP Professional 2010 and 2016 environments, and I’d welcome any corrections.
Dale Howard of Sensei Project Solutions has provided an excellent general examination of Calendars in Microsoft Project. It may prove useful to review his post before proceeding.
A. Project Calendar
The Project Calendar is used to schedule all tasks in a project IN THE ABSENCE OF OTHER CALENDARS. When present, Task Calendars supersede all of the Project Calendar’s functions, and Resource Calendars supersede some – but not all – of the Project Calendar’s functions.
Without Task or Resource Calendars, each task’s early start date occurs when all logic constraints have been satisfied and the Project Calendar makes work time available. The task’s early finish occurs when the assigned duration has been fully expended according to the Project calendar.
Relationship lags are computed according to the Project Calendar.
Start Slack, Finish Slack, and Total Slack are computed using the Project Calendar.
The default calendar for ProjDateAdd, ProjDateSub, and ProjDateDiff functions is the Project Calendar.*
Because only a single calendar is involved in all schedule calculations, Total Slack may be a reliable indicator of Critical Path within a single project schedule.
If two projects with different project calendars are joined together with inter-project dependencies, then the interaction of working periods between linked tasks can cause Total Slack to vary along a single driving logic path.
B. Project Calendar PLUS Resource Calendars
Each Resource possesses a unique Resource Calendar, which is comprised of a Base Calendar with specific modifications/exceptions. For example, the Base Calendar for all resources in a particular country may include standard weekends and holidays for that country. These are inherited by the Resource Calendar, while exceptions may be applied for specific Resource vacations. By default, the Base Calendar is the Project Calendar at the time the resource is created. An alternate Base Calendar can be assigned afterward. The Resource Calendar has the same name as the Resource.
When one or more resources are assigned to a task, the task is scheduled according to a) predecessor and successor logic, including lags; and b) the available working times in the Resource Calendars. The task’s early start date occurs when all logic constraints have been satisfied and at least one assigned resource has available work-time. The task’s early finish date occurs when the last resource assignment is completed – AND for Fixed-duration tasks with positive duration, the specified duration has been expended. For tasks that are not of type “Fixed Duration,” the Duration is the sum of all the intervals (from start to finish) during which at least one resource is working. Thus, a task with multiple resources (each with a unique calendar) may have a Duration and Start/Finish dates that do not directly correspond to ANY single defined Calendar. For Fixed-Duration tasks, the Duration is the difference between the early start and early finish as computed using the Project Calendar. Thus, a Fixed-Duration task with 12-hours of work by a night-shift resource can have a Duration of Zero, based on the Project’s Standard calendar. Moreover, a Fixed-Duration task with a specified duration of 2 days and 16 hours of work by a weekend-working resource may start on Saturday (when the resource is available) and not be completed until Tuesday evening, when its specified duration has been expended according to the project calendar. During the backward pass, Late dates are established similarly, based on (resource) working-time calendars.
Relationship lags are computed using the Project Calendar.
Start Slack, Finish Slack, and Total Slack are computed using the Project Calendar.
The default calendar for ProjDateAdd, ProjDateSub, and ProjDateDiff functions used in custom Task fields remains the Project Calendar. When used in custom Resource fields, the default calendar for these functions is the Resource’s Base Calendar, which is often the Project Calendar.*
Since a resource calendar may delay a task from starting work during an available work period as defined in the Project Calendar, the task’s driving predecessor may possess slack. Thus, Total Slack can vary along a single driving logic path.
C. Project Calendar PLUS Task Calendars (No Resource Calendars OR “Ignore Resource Calendars” Selected)
A task calendar may be created and assigned to multiple tasks. Each Task Calendar is a Base Calendar that may be created by copying and modifying an existing Base Calendar. (Because it is a base calendar itself, a task calendar does not inherit information from other calendars.)
Task Calendars may be used to refine schedule constraints based on the nature of the tasks being performed. E.g. seasonal or environmental limitations. Task Calendars may also be used to represent resource restrictions when no resources have been assigned (e.g. a year-end non-work period for certain tasks in a master/summary schedule.) When “Ignore Resource Calendars” is checked, then assigned Resources will be compelled to work exactly according to the Task Calendar, possibly violating their own work time availability.
Without effective Resource restrictions, the task’s early start date occurs when all logic constraints have been satisfied and the Task Calendar makes work time available. The task’s early finish occurs when the assigned duration has been fully expended according to the Task Calendar.
Relationship lags are computed according to the Task Calendar of the successor task, if it has one, or the Project Calendar.
Start Slack, Finish Slack, and Total Slack for each task are computed using the Task Calendar, if it has one, or the Project Calendar.
The default calendar for ProjDateAdd, ProjDateSub, and ProjDateDiff functions used in custom Task fields is the Task Calendar, if one exists, or the Project Calendar.*
The interval between a driving predecessor and a driven successor may possess work time according to the predecessor’s calendar but not the successor’s. The driving predecessor may possess slack. Thus, Total Slack can vary along a single driving logic path.
For most practical purposes, specifying a task duration using an “elapsed” unit (edays, for example), is essentially the same as: a) Applying a 24-hour task calendar with “ignore resource calendars” selected; AND b) Assigning a duration value that accounts for the project’s hours-per-day, hours-per-week, and days-per-month settings. For example, 1 elapsed day is the same as 24 hours or 3 “days” (8-hours each) applied to a 24-hour working calendar. (Since mixing duration “days” with 24-hour calendars routinely causes confusion, it is good practice to instead specify such durations in hours.)
Any task with an elapsed duration will have the Task Calendar field disabled. (A stored value may be visible, but it is inactive as long as the duration units are elapsed.)
Since elapsed-duration tasks automatically ignore resource calendars, any assigned Resources will be compelled to work 100% without rest, possibly violating their own work time availability. Consequently, it’s not a good idea to routinely apply elapsed durations together with resource loading. Even machines need downtime for maintenance.
Without effective Resource restrictions, the task’s early start date occurs when all logic constraints have been satisfied, period. The task’s early finish occurs when the elapsed duration has been fully expended.
Non-elapsed relationship lags are computed according to the Task Calendar of the successor task, if it has one, or the Project Calendar.
Start Slack, Finish Slack, and Total Slack for each elapsed-duration task are computed on the basis of elapsed time.
For tasks with elapsed durations, the default calendar for ProjDateAdd, ProjDateSub, and ProjDateDiff functions used in custom Task fields is the 24-Hour Calendar.*
The interval between an elapsed-duration predecessor and its driven (non-elapsed) successor may possess non-working time according to the successor’s effective calendar (task, resource, or project). The driving predecessor may possess slack. Thus, Total Slack can vary along a single driving logic path.
E. Project Calendar PLUS Task Calendars PLUS Resource Calendars (NOT “Ignored”)
If the task’s “Ignore Resource Calendars” box is NOT checked, then:
Each task is scheduled only during work time that is available in BOTH the Task Calendar and the applicable Resource Calendar for each assignment.
The task’s early start date occurs when all logic constraints have been satisfied, the Task Calendar makes work time available, AND at least one assigned resource has available work time. The task’s early finish occurs when the last assignment is completed within the combined work time restrictions.
Relationship lags are computed according to the Task Calendar of the successor task, if it has one, or the Project Calendar.
Start Slack, Finish Slack, and Total Slack are computed using the Task Calendar, if any, or the Project Calendar.
The default calendar for ProjDateAdd, ProjDateSub, and ProjDateDiff functions used in custom Task fields remains the Task Calendar, if one exists, or the Project Calendar. When used in custom Resource fields, the default calendar for these functions remains the Resource’s Base Calendar.*
As a result of either resource-delays or task calendar mismatches, Total Slack can vary along a single driving logic path.
* Note: The comparable Project VBA functions (Application.) DateAdd, DateSubtract, and DateDifference always default to the Project Calendar of the ActiveProject.
F. Slack and Calendars Re-Cap
In general, the Project Calendar of a fully resource-loaded project schedule plays no direct role in the calculation of the Early and Late dates, but it plays a primary role in MSP’s subsequent calculation of Slack based on those dates. Conversely, although resource calendars can fundamentally alter the logic-driven dates of a typical resource-loaded task, MSP ignores them in the Slack calculation. As a consequence, both the calculation and interpretation of Total Slack in a resource-loaded schedule become greatly simplified, if sometimes misleading.
Alternately, whenever a task calendar is applied (with or without resource-loading), that same calendar is used to calculate the Dates AND the Slack. Consequently, the calculation of Total Slack seems to be more correct and can be equally simple to calculate (using a Task- rather than Project-Calendar), but its interpretation can be confusing.
For example, the chart below illustrates two alternate methods for modeling a calendar-restricted Board-approval activity in a project schedule. The Board meets on the third Wednesday of each month for, among other items, approving key project commitments. If the project team fails to prepare the necessary documents in sufficient time for the meeting, then the approvals (and follow-on tasks) will be delayed by a month. (This is exactly how project governance works in some organizations.) For this example, the board-approval, preparation, and follow-up activities are not on the Critical Path for the project, finishing up about a month before the project’s finish milestone.
In the first case, the restraint on the Board Approval task is modeled by applying a Task Calendar with only the third Wednesday of each month as a working day. In the second case, the restraint is modeled by loading a “Board Availability” resource whose Base Calendar is exactly the same as the Task Calendar applied above. Early Dates and Late Dates for all tasks are identical for both cases, and the only difference is the Total Slack of the Board Approval task. This value is computed as the difference between the task’s Late Finish (17Apr’19) and its Early Finish (20Mar’19). When the restraint is applied using the Task Calendar, the Total Slack of 1 day reflects the fact that one Board Meeting/availability day exists between the two dates. With the restraint applied using a resource calendar, the Project Calendar applies, and Total Slack of 20 days reflects the twenty weekdays between the two dates.
In either case, the example also illustrates the difficulty of identifying logic paths using Total Slack alone.
G. A Note on the Resource Availability Grid
The Resource Availability Grid (part of the Resource Information dialog window) is sometimes seen as an alternate/supplemental method for specifying resource working time. Unlike the Resource Calendar, however, Resource Availability entries do not participate in the working-time definitions that drive the scheduling calculations. Rather, they serve as a time-phased version of the Max Units property for identifying over-allocation of resources. Once flagged, MSP can attempt to resolve these over-allocations through automatic resource-leveling. This is distinct from logic-driven scheduling.
The “critical” activity flags in modern project schedules often do not correctly identify the true critical paths. Blind acceptance of such “critical” flags to identify the critical path inhibits proper understanding, communication, and management of project schedule performance – and gives CPM a bad rap.
Basic CPM Concepts (in General):
The “critical path method” (CPM) – a ~60-year-old algorithm of fairly straightforward arithmetic – lies at the core of most modern project scheduling tools, and most project managers worthy of the name have been exposed to at least the basic CPM concepts. Any discussion of the critical path must address the underlying conceptual basis:
A CPM project schedule is comprised of all the activities necessary to complete the project’s scope of work.
Activity durations are estimated, and required/planned sequential restraints between activities are identified: e.g. Predecessor task “A” must finish before successor task “B” can start, and predecessor task “C” must finish before successor task “D” can start. The combination of activities and relationships forms a schedule logic network. Below is a diagram of a simple schedule logic network, with activities as nodes (blocks) and relationships as arrows.
Logic Relationships. A logic relationship represents a simple (i.e. one-sided) schedule constraint that is imposed on the successor by the predecessor. Thus, a finish-to-start (FS) relationship between activities A and B dictates only that the start of activity B may NOT occur before the finish of activity A. (It does not REQUIRE that B start immediately after A finishes.) Other relationship types – SS, FF, SF, which were added as part of the precedence diagramming method (PDM) extension of traditional CPM – are similarly interpreted. E.g. A–>(SS)–>B dictates only that the start of B may not occur before the start of A. Activities with multiple predecessor relationships must be scheduled to satisfy ALL of them.
Logic Paths. A continuous route through the activities and relationships of the network – connecting an earlier activity to a later one – is called a “logic path.” Logic paths can be displayed – together or in isolation – to show the sequential plans for executing selected portions of the project. The simple network shown has only two logic paths between the start and finish milestones: Path 1 = (StartProject) <<A><B>> (FinishProject); and Path 2 = (StartProject) <<C><D>> (FinishProject). [Experimenting with some shorthand logic notation: “<” = logic connection to activity’s Start; “>” = logic connection to activity’s Finish.]
Schedule Calculations. Schedule dates are calculated using three essential steps:
During the forward pass, the earliest possible start and finish dates of each activity are computed by considering the aggregated durations of its predecessor paths, beginning from the project start milestone and working forward in time.
Assuming an implicit requirement to finish the project as soon as possible, the early finish of the project completion milestone is adopted as its latest allowable finish date. This can be called the finish reflection. (Most CPM summaries ignore this step. I include it because it is the basis for important concepts and complications to be introduced later.)
During the backward pass, the latest allowable start and finish dates of each activity are computed by considering the aggregated durations of its successor paths, beginning from the project completion milestone and working backward in time.
Driving and Non-Driving Logic. A logic relationship may be categorized as “driving” or “non-driving” depending on its influence over the early dates of the successor activity – as calculated during the forward pass. A driving relationship controls the early start/finish of the successor; a non-driving relationship does not. In other words, a “driving” relationship prevents the successor activity from being scheduled any sooner than it is. A logic path (or path segment) may be categorized as “driving” (to its terminal activity) when all of its relationships are driving. [Such a path is sometimes called a “string.”]
Total Float. In simplified terms, the difference between the early start/finish and late start/finish of each activity is termed the activity’s “total float” (or “total slack”). A positive value denotes a finite range of time over which the activity may be allowed to slip without delaying “the project.” A zero value (i.e. TF=0) indicates that the activity’s early dates and late dates are exactly equal, and any delay from the early dates may delay “the project.” It is important to remember that total float/slack is nominally computed as a property of each individual activity, not of a particular logic path nor of the project schedule as a whole. [While computed individually for each activity, the float is not possessed solely by that activity and is in fact shared among all the activities within a driving logic path. In the absence of certain complicating factors, it is common to refer to a shared float value as a property of that path.]
Critical Path. A project’s critical path is the path (i.e. the unique sequence of logically-connected activities and relationships) that determines the earliest possible completion of “the project.” I prefer to call this the “driving path to project completion.” Other logic paths through the schedule are considered “near-critical paths” if they are at risk of becoming the critical path – possibly extending the project – at some time during project execution. In our simple project shown below, the critical path is Path 1, whose total duration of 4 weeks (20 days on a standard 5dx8h calendar) controls the early finish of the completion milestone.
In unconstrained schedule models incorporating only a single calendar (and without other complicating factors), the finish reflection causes the activities on the critical path to have late dates equal to their early dates; i.e. TF = 0. Consequently, any delay of a critical-path activity cascades directly to delay of the project completion. The near-critical paths are then defined as those paths whose activities have TF more than zero but less than some threshold. In traditional “critical path management,” activities that are NOT on or near the critical path may be allowed to slip, while management attention and resources are devoted to protecting those activities that are on or near the critical path. More importantly, acceleration of the project completion (or recovery from a prior delay) may only be accomplished by first addressing the activities and relationships on the critical path.
[Note: The definition of “critical path” has evolved with the introduction of new concepts and scheduling methods over the years. The earliest definitions – based on robust schedule networks containing only finish-to-start relationships, with no constraints, no lags, and no calendars – were characterized by the following common elements:
It contained those activities that determined the overall duration of the project (i.e. the “driving path to project completion.”)
It contained those activities that, if allowed to slip, would extend the duration of the project (hence the word “critical”).
A delay of any of its activities would be directly transmitted to an equal (matching) delay of the project completion.
Its activities comprised the “longest path” through the schedule network. That is, the arithmetic sum of their durations was greater than the corresponding sum for any other path in the network.
After completion of the forward and backward passes, its activities could be readily identified by a shared total float value of zero. Thus TF=0 became the primary criterion for identifying the critical path.
With the incorporation of non-FS relationships, early and late constraints, lags, and calendars in modern project scheduling software, these observations are no longer consistent with each other nor sometimes with a single logic path. Some of these inconsistencies are addressed later in this article. Only the first of these defining elements (“driving path to project completion”) has been generally retained in recent scheduling standards and guidance publications, though implied equivalence of the others continues to persist among some professionals.]
Software – the Critical Activities / Critical Tasks:
The basic element of modern project schedules is the activity or task. In most scheduling tools, logic paths are not explicitly defined. Nevertheless, the obvious importance of the critical path dictates that software packages attempt to identify it – indirectly– by marking activities that meet certain criteria with the “critical” flag. Activities with the “critical” flag are called “critical activities” (or “critical tasks”) and are typically highlighted red in network and bar-chart graphics.
Applying Critical Flags using Default Total Float Criteria
The simplest criterion for flagging a task as “critical” is TF=0. This is the primary method that most new schedulers seem familiar with, and it is the default criterion for some software packages. As noted earlier, this criterion is applicable to schedules with no constraints and only a single calendar. In Microsoft Project (MSP) and Oracle Primavera P6 (P6), the default “critical” flag criterion is TF<=0, and the threshold value of “0” can be adjusted. The differences between these criteria and the simpler TF=0 criterion are justified by four primary concerns:
Risk Management. Due to the inherent uncertainty of activity duration estimates, the critical path of a real-world project schedule – as ultimately executed – often includes an unpredictable mix of activities from the as-scheduled critical path and near-critical paths. In the absence of quantitative schedule risk assessment, it is reasonable to consider all such (potentially-critical-path) activities equally when evaluating project schedule risks. This purpose is easily served by applying the “critical” flag to all activities whose TF value is less than or equal to some near-critical threshold.
Late Constraints. Overall project completion priorities (and contractual requirements) often lead to the imposition of deadlines (in MSP), late-finish constraints (in MSP and P6), or project constraints (in P6). Such constraints can override the finish reflection and cause the late dates of some activities to be earlier or later than they would be in the absence of the constraints. As a result, total float can vary among the activities on the driving path to project completion. In a project with multiple constrained milestones, the driving path to only one of them (the most “urgent”) can be expected to have a constant total float value (i.e. the lowest total float.) Due to intersecting logic paths, total float can vary along the driving paths to other constrained milestones. Applying the “critical” flag to activities with total float less than or equal to the project’s lowest total float marks those activities that are on the driving path to the most urgent constrained milestone in the project. If a project constraint (in P6 only) is applied, the lowest total float value may be greater than zero; without a more urgent constraint, the marked activities then denote the driving path to the final activity in the project.
Negative Float. Late constraints can cause late dates to precede early dates for certain activities. This results in negative values for total float/slack (i.e. TF<0). In practically all cases, negative total float indicates that the activity cannot be scheduled in time to satisfy one or more of the deadlines or constraints (though which one of these is violated may not be clear); and some corrective action is necessary. [*The concept of negative float – and the constraints that create it – were not included in the foundations of CPM and PDM. Negative float is not universally accepted among scheduling professionals today, and not all scheduling software supports its calculation.]
Applying the “critical” flag to all activities with total float less than or equal to zero then marks all activities that:
Are on the driving path to an unconstrained project completion (i.e. TF=0, controlled by the project’s finish reflection); or
Are on the driving path to a constrained project completion or intermediate milestone that is just barely met (i.e. TF=0, controlled by deadline/constraint); or
Are on the driving path to project completion where an explicit project completion milestone is violated (i.e. TF<0, controlled by project deadline/constraint); OR
Are on the driving path to some intermediate activity whose constraint is violated (i.e. TF<0, controlled by intermediate deadline/constraint); or
Are on any number of non/near-driving paths to one or more constrained project completion or intermediate milestones, (i.e. TF<0). Though non-driving, these paths must still be shortened (in addition to shortening the driving and nearer-driving paths) to meet the milestones.
Working-Time Calendar Effects. When activities with different calendars are logically connected in a schedule network, the interval between the finish of a predecessor task and the start of its successor may sometimes contain working time for the predecessor but not for the successor. If this occurs, then a driving logic relationship exists, but the predecessor still has room to slip without delaying any other tasks or the project (i.e. it possesses float.) Thus, total float may vary along a single driving logic path, including the critical path. The amount of this variation depends on the size of potential offsets between calendars: from a few hours (for shift calendar offsets) to a few days (for 5-day and 7-day weekly calendars offsets) to a few months (for seasonal-shutdown calendar offsets).
Applying the “critical” flag to all activities with total float less than or equal to the largest calendar-related offset will mark all activities that:
Are on the driving path to project completion with TF<=0;
Are on the driving path to project completion but with TF>0 (and less than the specified offset);
Are NOT on the driving path to project completion but have TF less than the specified offset. These are false positives. For these activities, total float could be controlled either by the finish reflection (TF>=0) or by some other constraint.
Critical Flags and Critical Paths
Unfortunately, applying the “critical” flag as noted for most of these considerations has one consistent result: the continuous sequence of activities and relationships constituting a “critical path” often remains obscured. It is disappointing that the majority of project schedulers – using MSP or P6 – continue to issue filtered lists of “critical” activities as “the critical path.” Much of the time – especially in MSP – they are not. Even among expert schedulers, there is a persistent habit of declaring total float as the sole attribute that defines the critical path rather than as a conditional indicator of an activity’s presence on that path.
When an activity is automatically marked “critical” based on total float/slack, the primary conclusion to be drawn is simply, “this activity has total float/slack that is at or below the threshold value. That is, there is insufficient working time available between the early- and late- start/finish dates.” If total float/slack is less than zero, then one might also conclude, “this activity is scheduled too late to meet one or more of the project’s deadlines/constraints.” [If automatic resource leveling has been applied, then even these simple conclusions are probably incorrect.] These are important facts, but a useful management response still requires knowledge of the driving logic path(s) to the specific activities/milestones whose deadlines/constraints are violated – knowledge that total float/slack and its associated “critical” flag do not always provide.
Workarounds for Total Float Criteria
P6 provides several features, not available out-of-the-box in MSP, for correctly identifying the critical path when total float criteria do not. Specifically:
For Risk Management. P6’s multiple-float path analysis (MFP) allows the identification of successive driving and near-driving paths to specified project completion milestones. Monitoring progress on these paths is worthwhile for risk management. I’ve previously written about MFP analysis HERE. P6 does not support using float paths (the output of MFP analysis) as an explicit criterion for the “critical” activity flag.
For Late Constraints and Negative Float. P6 allows a negative critical float threshold. It is possible to set this threshold low enough so that only the “path of lowest total float” is marked as critical. In the absence of working time calendar effects, this criterion can be effective in identifying the (most) critical path. Thus it is possible to correctly identify the project’s critical path when: a) there is only a single constraint on the project; and b) that constraint coincides with the sole project completion milestone; and c) that constraint is violated (creating negative float).
MSP does not allow a negative critical float threshold, so correct identification of the critical path in a negative float scenario is not possible. All tasks with negative total slack are automatically and unavoidably flagged as “critical.”
If the P6 schedule has a project “must finish by” constraint, then the activities on the critical path may have positive total float. In that case, the lowest-float criterion may be applied (using a positive threshold) to correctly identify the critical path.
For Working-Time Calendar Effects. Unlike other project scheduling software, P6 allows the “critical” activity flag to be assigned on the basis of some criterion other than total float – called Longest Path. The name is misleading, as the method is based on driving logic rather than activity durations. Any activity that is found on the driving logic path to project completion is flagged as “critical.” (The algorithm tracks driving logic backward from the task(s) with the latest early finish in the project.) The Longest Path criterion ignores the total float impacts of multiple calendars and constraints. While it is effective in identifying the project’s critical (logic) path, Longest Path alone is not useful for identifying near-critical paths. MFP analysis (noted above) is useful for this purpose. “Longest path value ™,” a relative-float metric available in Schedule Analyzer Software (a P6 add-in) also helps to identify near-critical paths in these circumstances. For a more detailed review, see What is the Longest Path in a Project Schedule?
MSP provides no out-of-the-box solutions to address these weaknesses in critical path identification. Total float/slack remains the sole basis for applying the “critical” flag, yet the impacts of constraints, deadlines, and calendars remain unaddressed. In MSP 2013 and later versions, the “task path” bar style modifier does provide a basis for graphically identifying the driving path to a selected completion activity, and this is helpful. Nevertheless, a logic tracing add-in (like the BPC Logic Filter program that I helped to develop) is necessary to correctly identify the controlling schedule logic – including the true critical path – in a complex MSP schedule.
Definitions and Recommended Practices
Defense Contract Management Agency (DCMA – 2009)
DCMA’s in-house training course, Integrated Master Plan/Integrated Master Schedule Basic Analysis (Rev 21Nov09) is the source of the “14-Point Assessment” that – because its explicit “trigger” values are easily converted to pass/fail thresholds and red/yellow/green dashboards – is seen as a de-facto industry standard for schedule health assessment. The course materials contain the following definitions:
(Slide 28) Critical Path ~ Sequence of discrete work packages that has the longest total duration through an end point. ~ has the least amount of total float ~ cannot be delayed without delaying the completion date of the contract (assuming zero float). (Slide 98) Critical Path – Definition: a sequence of discrete tasks/activities in the network that has the longest total duration through the contract with the least amount of float. ~ A contract’s critical path is made up of those tasks in which a delay of one day on any task along the critical path will cause the project end date to be delayed one day (assuming zero float). (Slide 99) The critical path is ‘broken’ whenever there is not a sequence of connected critical path tasks that goes from the first task of the schedule until the last task. A broken Critical Path is indicative of a defective schedule.
These definitions are mostly (though not entirely) consistent with each other. They do share a common emphasis on the … “longest”… “sequence” … with “lowest total float” and direct transmission of delay from any critical-path task directly to the project’s completion. Obviously, the reliance on total float makes them incompatible with any project schedule that incorporates multiple calendars, late constraints, or resource leveling.
(Slide 97) Critical Task: Some tasks possess no float…they are known as critical tasks. ~Any delay to a critical task on the critical path will cause a delay to the project’s end date.
Unlike most of the later definitions, DCMA’s appears to contemplate the existence of critical tasks that are not on the critical path. Obviously, the expectation that such critical tasks possess “no float” is not compatible with negative-float regimes, nor is it compatible with the positive-float regimes that accompany project “must finish by” constraints in P6.
AACE International (2010 & 2018)
AACE International (formerly the Association for the Advancement of Cost Engineering) maintains and regularly updates its Recommended Practice No. 10S-90: Cost Engineering Terminology. The most recent issue of RP 10S-90 (June 2018) includes the following definitions:
CRITICAL PATH – The longest continuous chain of activities (may be more than one path) which establishes the minimum overall project duration. A slippage or delay in completion of any activity by one time period will extend final completion correspondingly. The critical path by definition has no “float.” See also: LONGEST PATH (LP). (June 2007)
CRITICAL ACTIVITY – An activity on the project’s critical path. A delay to a critical activity causes a corresponding delay in the completion of the project. Although some activities are “critical,” in the dictionary sense, without being on the critical path, this meaning is seldom used in the project context. (June 2007)
Unfortunately, these definitions fall apart in the presence of multiple calendars, multiple late constraints, or negative total float – when the second and third clauses in both definitions no longer agree with the first. They appear distinctly out of sync with modern project scheduling practices, and (according to AACE International’s Planning and Scheduling Subcommittee Chair) an update is pending.
AACE International’s RP No. 49R-06, Identifying the Critical Path (last revised in March 2010) instead defines the Critical Path as
…the longest logical path through the CPM network and consists of those activities that determine the shortest time for project completion. Activities within this [group (sic)] or list form a series (or sequence) of logically connected activities that is called the critical path.
Aside from the apparently inadvertent omission of a word, I don’t have any problem with this definition. It is certainly better, in my opinion, than the first.
RP 49R-06 notes the existence of “several accepted methods for determining the critical path” and goes on to describe the four “most frequently used” methods:
Lowest Total Float. This is as I described under Workarounds for Total Float Criteria, above. Although this method is listed first, the RP spends four pages detailing the issues that make total float unreliable as a CP indicator. As long as the CP is to be defined only with respect to the most urgent constraint in the schedule (including the finish reflection) – and there are no calendar issues – then this method provides a useful result.
Negative Total Float. In apparent acquiescence to the limitations of MSP, the RP describes this method by first abandoning the fundamental definition of the critical path as a specific logic path. It then allows the “critical” classification for any activity that must be accelerated in order to meet an applied deadline or constraint. Ultimately, the RP attempts to justify this method based solely on certain legal/contractual considerations of concurrent delay. It is not useful for those whose primary interest is timely completion of the project, or a particular part of the project, using critical path management principles.
Longest Path. This “driving path to project completion” algorithm, as I described above in Workarounds for Total Float Criteria, has been implemented in versions of (Oracle) Primavera software since P3 (2.0b). It is the preferred method for P6 schedules with constraints and/or multiple activity calendars. A similar algorithm is included in BPC Logic Filter, our add-in for Microsoft Project. While the method is nominally aimed at finding the driving path(s) to the last activity(ies) in the schedule, it can be combined with other techniques (namely a super-long trailing dummy activity) to derive the driving path to any specific activity, e.g. a specific “substantial completion” or “sectional-completion” milestone.
“Longest Path Value.” This is an expanded method for identifying the driving and near-driving paths to project completion. The method works by adding up relationship floats leading to a specific substantial completion milestone. If the aggregate value of these floats along a specific logic path (i.e. “Longest Path Value”) is zero, then that path is identified as the critical path. While the RP suggests that this method can be performed manually (presumably by “click-tracing” through the network of a P6 schedule), manual implementation in complex schedules is tedious and error prone. As implemented in Schedule Analyzer Software, this method is essentially an improved version of P6’s Longest Path method (except that the add-in cannot change the “critical” flag for activities.) It is a preferred method in P6 for those possessing the Schedule Analyzer Software. BPC Logic Filter performs similar analyses – using “path relative float” instead of “Longest Path Value” – for MSP schedules.
While not listed among the “most frequently used” methods, P6’s MFP analysis option is briefly addressed by the RP in the context of identifying near-critical paths. BPC Logic Filter performs similar analyses for MSP schedules.
None of the four methods described are useful for identifying the resource critical path (or resource-constrained critical path) of a leveled schedule.
Project Management Institute (PMI-2011)
PMI’s Practice Standard for Scheduling (Second Edition, 2011) explicitly defines the critical path as…
Generally, but not always, the sequence of schedule activities determining the duration of the project. Generally, it is the longest path through the project. However, a critical path can end, as an example, on a schedule milestone that is in the middle of the schedule model and that has a finish-no-later-than imposed date schedule constraint.
Unlike the RP (49R-06) from AACE International, PMI’s Practice Standard provides no meaningful method for quantitatively identifying the activities of the critical path (or any logic paths) in a particular schedule model. In fact, in its description of the precedence diagram method (PDM – the modern version of CPM used by most modern scheduling software) the Practice Standard acknowledges the complicating factors of constraints and multiple calendars but notes that “today’s computerized scheduling applications complete the additional calculations without problems.” Then it concludes, “In most projects the critical path is no longer a zero float path, as it was in early CPM.” The Practice Standard goes on to scrupulously avoid any explicit link between total float and the critical path. The impact of all this is to just take the software’s word for what’s “critical” and what isn’t. That’s not particularly helpful.
Finally, educating senior stakeholders on the subtle difference between “schedule critical” and “critical” is always one of the first issues faced when implementing systematic project management in non-project focused organizations. The Practice Standard’s several conflicting definitions of critical activities tend to confuse rather than clarify this distinction.
U.S. Government Accountability Office (GAO-2015)
The GAO’s Schedule Assessment Guide: Best Practices for Project Schedules (GAO-16-89G, 2015) has been taken to supersede the earlier DCMA internal guidance in many formal uses. (Nevertheless, the GAO’s decision to discard any formal trigger/threshold values – a good decision in my view – means that the DCMA-based assessments and dashboards remain popular.) The GAO document contains the following formal definitions:
Critical path: The longest continuous sequence of activities in a schedule. Defines the program’s earliest completion date or minimum duration.
[With some minor reservations related to meaning of “longest,” I believe this is a good definition.]
Critical activity: An activity on the critical path. When the network is free of date constraints, critical activities have zero float, and therefore any delay in the critical activity causes the same day-for-day amount of delay in the program forecast finish date.
[Unfortunately, the caveats after the first clause are insufficient, ignoring the complicating effects of multiple calendars.]
For the most part – and despite the float-independent formal definition above – the Schedule Assessment Guide’s “Best Practices” tend to perpetuate continued reliance on total float as the sole indicator of the critical path. In fact, “Best Practice 6: Confirming That the Critical Path Is Valid” does a good job of illustrating the complicating factors of late constraints and multiple calendars, but this review leads essentially to the differentiation of “critical path” (based on total float alone) from “longest path” (based on driving logic). This is a direct contradiction of the formal definition above. In general, the text appears to be written by a committee comprised of P6 users (with robust driving/Longest Path analysis tools) and MSP users (without such tools.) Thus, for every “longest path is preferred,” there seems to be an equal and opposite, “the threshold for total float criticality may have to be raised.” This is silly.
National Defense Industrial Association (NDIA-2016)
The NDIA’s Integrated Program Management Division has maintained a Planning & Scheduling Excellence Guide (PASEG), with Version 3.0 published in 2016. The PASEG 3.0 includes the following key definitions:
Critical Path: The longest sequence of tasks from Timenow until the program end. If a task on the critical path slips, the forecasted program end date should slip.
Driving Path(s): The longest sequence of tasks from Timenow to an interim program milestone. If a task on a Driving Path slips, the forecasted interim program milestone date should slip.
The second clause of each definition – which presumes a single calendar – is included in the Schedule Analysis chapter but is excluded from the formal definition in Appendix A. Timenow is effectively the data/status date. The PASEG does not define or mention critical task/activity as distinct from a “task on the critical path.”
The PASEG notes, “Some of the major schedule software tools have the ability to identify and display critical and driving paths. Additionally, there are many options available for add-in/bolt-on tools that work with the schedule software to assist in this analysis.” [I suppose BPC Logic Filter would be one of the mentioned add-in tools for Microsoft Project.]
The PASEG also mentions some manual methods for identifying critical and driving paths, e.g.:
a. Imposing a temporary, super-aggressive late constraint and grouping/sorting the output (presumably by total float and early start. Though not explicitly mentioned in the method description, total float is the key output affected by the imposed constraint.) Obviously, this method isn’t reliable when more than one calendar is used.
b. Building a custom filter by manually “click-tracing” through driving logic and marking the activities. This method is most reliable in P6, with some caveats. It is reliable in MSP only under some fairly restrictive conditions.
In general, these methods are non-prescriptive, though the emphasis on driving logic paths (rather than total float) seems clear.
Guild of Project Controls (GPC, “The Guild” – 2018)
The Guild is a relatively young (~2013) international community of project controls practitioners – initially associated with the PlanningPlanet.com web site – whose founding members have assembled a Project Controls Compendium and Reference (GPCCaR). The GPCCar takes the form (more or less) of an introductory training course on Project Controls, including Planning and Scheduling. The GPCCaR includes no formal Glossary, Terminology, or Definitions section, so “critical path” and “critical path activities” accumulate several slightly varying definitions in the applicable Modules (07-01, 07-7, and 07-8). In general, “zero total float” and “critical path” are used interchangeably, and the complications of multiple calendars and multiple constraints in P6 and MSP are ignored. This is not a suitable reference for complex projects that are scheduled using these tools.
American Society of Civil Engineers (ASCE)
ASCE Standard ANSI/ASCE/CI 67-17 – Schedule Delay Analysis is one of the few documents with a clear and correct distinction between the critical path and the collection of critical activities:
Critical path—The series of logically connected tasks that define the minimum overall duration for completion of the project, also known as the longest path. There can be more than one critical path in the schedule.
Critical activities—Activities with zero or negative float in a schedule reflecting a current adjusted completion date, some of which may not be on the critical path.
A full understanding of driving and non-driving schedule logic paths for major schedule activities is useful for managing and communicating a project execution plan.
The most important logic path in the project schedule is the “critical path,” i.e. the driving path to project completion. Overall acceleration (or recovery) of a project is only made possible by first shortening the critical path. Acceleration of activities that are not on the critical path yields no corresponding project benefit to project completion. Multiple critical paths may exist.
Some traditional notions of critical path path attributes – e.g. critical path activities possess no float; slippage or acceleration of critical path activities always translates directly to project completion – are not reliable in modern project schedules.
Total float remains a valuable indicator of an activity’s scheduling flexibility with respect to completion constraints of the project. An activity with TF=0 may not be allowed to slip if all project completion constraints are to be met. Activities with TF<0 must be accelerated if all the constraints are to be met.
Project scheduling software typically defines individual activities as “critical” without fully accounting for common complicating factors like multiple constraints and calendars. As a result, the collection of “critical” tasks/activities in a complex project schedule often fails to identify a true critical path.
A critical task/activity is best defined (in my opinion) as either:
An activity that resides on the critical path; or
An activity whose delay will lead to unacceptable delay of the project completion; or
An activity whose delay will lead to unacceptable delay of some other constrained activity or milestone.
In general, these conditions are mutually exclusive, and different activities within a single project schedule may satisfy one or more of them.
Professional project managers and schedulers should be careful not to automatically characterize “critical” tasks (i.e. those with low total float) as indicators of a project’s critical path when complicating factors are present.
In Oracle Primavera P6, Multiple Float Path analysis is useful for identifying and organizing logic paths leading to a selected End activity in a schedule. If the Driving Logic to the End activity is desired, then the Free Float option should be selected.
When I wrote about P6’s Multiple Float Path (MFP) analysis here, I suggested using the Free Float option for identifying driving logic paths. Since then, I’ve encountered more than a few professionals who believe that the Total Float option also identifies driving logic. This entry provides a simple example illustrating why that is not always the case.
This “Testing Project – MFP” is a simple project that includes no constraints and only a single calendar (5dx8h). Both “Longest Path” and “Total Float” criteria lead to the same Critical Path: A-B-C-D-E-FINISH. There are several non-critical branches from the Critical Path: namely A1, B1, and C1 are successors to A, B, and C respectively. C1 is the start of the non-critical branch: C1-C2-C3. It has both A1 and B1 as predecessors, and it is driven by B1.
For this project, it is obvious (and a trivial exercise to demonstrate) that the driving logic path for any Critical Path activity is comprised of its “Critical” predecessors; i.e. those activities that are predecessors of the selected End activity and which have zero Total Float (TF=0).
What if we are primarily interested in the driving logic to an End activity that is NOT on the Critical Path – activity C3 for example? By simple inspection of the schedule (or by click-tracing from C3 backward through “driving” relationships), it is easy to see that C3’s driving logic path is comprised of the following activities: A-B-B1-C1-C2-C3. Since Total Float varies along this path (0-0-17-17-17-17), it is clear that driving logic for C3 is not associated with Total Float.
Another way to examine the logic controlling activity C3 is to re-schedule the project while calculating multiple float paths. MFP analysis examines the predecessor activities leading to a selected End activity (C3 in this case) in order of their logic sequence and assigns each of these (excluding LOE activities) to a numbered Float Path. Float Path 1 is “the most critical path.” The analysis stops when the specified number of float paths is reached. Afterward, organizing the schedule by Float Path and sorting by Float Path Order leads to a clear differentiation of logic sequence paths. I also routinely filter-out activities without float path assignment and activities with ALAP constraints. The construction of the various float paths is governed by which float option is selected – either Total Float or Free Float – in the advanced schedule options.
Total Float Option
This is what the P6 Help file says for the Total Float option.
Total Float – Choose this option to identify critical paths based on the total float of activity relationships. To calculate the most critical path, the module first determines which relationship has the most critical total float. Using this relationship as the starting point, the module determines which predecessor and successor activities have the most critical relationship total float, among all possible paths, until an activity is reached that does not have any relationships. The path that contains these activities is the most critical path.
Using the Total Float option and the other parameters shown for our simple project leads to the result shown below. Float Path 1 is limited to those activities that 1) are predecessors of C3, AND 2) have a Total Float of 0. According to P6, this is “the most critical path.” Float Path 2 is comprised of C3’s predecessors (and C3 itself) that have a Total Float of 17. Float Path 3 comprises the single logical predecessor of C3 with TF = 24. Thus, float paths appear to correspond to Total Float alone.
[Sep’19 Edit] I’ve recently read the only public foundation document for the Total Float algorithm – originally called “Enhanced” PDM in 2004 – and the observed behavior is as expected according to that algorithm. Essentially, Float Path 1 is “seeded” by whichever path-predecessor of C3 has the lowest total float (subject to some tie breakers.) Once seeded, the path is defined/traced by “bi-directional driving” relationships from the seed point. A “bi-directional driving” relationship exists when:
The Relationship Total Float equals the total float of the predecessor activity; AND
Relationship Successor Free Float = 0. (P6 support docs point to the “most critical” Relationship Successor Total Float here, but that didn’t stand up to close scrutiny.)
Subsequent float paths are seeded and traced using the same priorities.
Fundamentally, the algorithm was developed to a) differentiate float-based critical paths and near-critical paths in the presence of multiple calendars; and b) differentiate independent driving logic paths, including multiple critical paths, that share the same total float. So here in the absence of calendars or parallel critical paths, the alignment of the calculated float paths and total float is exactly as expected.
Clearly, none of the 3 float paths from the Total Float option correspond to the actual Driving Path to activity C3. Path 1 includes activity C, which although it is on the Project’s Critical Path is NOT on the Driving Path to activity C3. The actual Driving Path has been split between Float Path 1 and Float Path 2.
Thus, using Total Float option, Float Path 1 – “the most critical path” – comprises those activities that are predecessors of activity C3 and have the lowest total float. C3’s own driving/controlling logic path is not defined by the float paths assigned.
Free Float Option
This is what the P6 Help file says for the Free Float option.
Free Float – Choose this option to define critical float paths based on longest path. The most critical path will be identical to the critical path that is derived when you choose to define critical activities as Longest Path in the General tab. In a multicalendar project, the longest path is calculated by identifying the activities that have an early finish equal to the latest calculated early finish for the project and tracing all driving relationships for those activities back to the project start date. After the most critical path is identified, the module will calculate the remaining sub-critical paths.
Using the Free Float option and the other parameters shown for our simple project leads to the result shown below. Float Path 1 exactly corresponds to the known Driving Path to activity C3: i.e. A-B-B1-C1-C2-C3. Float Path 2 is comprised of activity C only, while Float Path 3 is comprised of activity A1 only. Float paths clearly have no correspondence to Total Float.
The key decision point in allocating float paths seems to occur at the predecessors to activity C1: i.e. C, A1, and B1.
C has an activity Total Float of 0. The C-C1 relationship has a Relationship Total Float of 20 and a Relationship Free Float of 3.
A1 has an activity Total Float of 24. The A1-C1 relationship has a Relationship Total Float of 24 and a Relationship Free Float of 7.
B1 has an activity Total Float of 17. The B1-C1 relationship has a Relationship Total Float of 17 and a Relationship Free Float of 0.
Although the Help file is essentially silent on the issue, the MFP analysis appears to allocate these predecessor activities to the three float paths on a basis that correlates to Relationship Free Float. Here, a Relationship Free Float of zero indicates a Driving Relationship. Successively higher values of relationship free float correspond to less-driving relationships and result in assignment to higher-numbered float paths.
Thus, using Free Float option, Float Path 1 – “the most critical path” – comprises the driving path to the selected end activity. Higher-numbered float paths correspond to “sub-critical” paths, or to successively less-driving paths to the selected end activity.
The “Critical Path” of a logic driven project schedule is the collection of activities that determine the earliest possible completion date of the project – i.e. the driving logic path to project completion. In the original Critical Path Method and its variants, the Critical Path was reliably correlated to a Total Float value of zero, and delay (or acceleration) of any Critical Path activity cascaded directly to the project completion milestone. Near-critical paths were defined by successively higher values of Total Float. In simple projects, therefore, Total Float is a reliable indicator of the logical association between any given activity and the project’s Completion.
Because of Total Float’s significance in the traditional definition of Critical and Near-Critical Paths, it is easy – but generally incorrect – to presume a logical association between two activities on the basis of their Total Float values. In the absence of any late constraints, multiple calendars, or resource leveling, then such associations may exist between certain critical or lower-float successors and their higher-float predecessors. Thus, running MFP analysis using the Total Float option may be expected to reveal driving and near-driving logic when the selected End activity is on the float-defined Critical Path. As shown in the example above, however, such an analysis does not reveal driving and near-driving logic when the selected End activity is not Critical.
When a project schedule includes multiple calendars, resource leveling, or a late constraint on any activity except the final one, then Total Float becomes unreliable for indicating the driving path to project completion. Similarly, it becomes less useful for identifying driving and near-driving logic paths to selected activities even when they are on the “Critical Path”. In projects with multiple calendars and modest progress updates, the float paths defined using the Total Float option can deviate substantially from both the known driving paths and simple Total Float-based paths. Under these conditions, the Free Float option is almost certain to provide a clearer view of the schedule logic driving an activity, regardless of its criticality.
In complex project schedules, multiple critical paths can exist between the project’s start and finish milestones. Additionally, integrated projects with multiple phased scopes of delivery often have several distinct, contractually-mandated deliverables and corresponding delivery dates. Each possesses its own critical/driving path.
[Note to searchers: This article has a slight mention of Microsoft Project’s Advanced Calculation Option – “Calculate Multiple Critical Paths.” It’s near the end, in the section called A Note About Open Ends.]
A key tenet of the original Critical Path Method (CPM) of project scheduling is that each project has one and only one “Critical Path” (CP) that extends continuously from the project start milestone to the project finish milestone. The CP is defined by the collection of activities that determine the finish date of the project, such that a delay of any one of them will delay the project. Traditional methods identify the CP based on Total Slack/Float. [Note: in many cases, the CP is not defined by the collection of “Critical” activities from the software.]
Single Finish Milestone w/ Parallel Drivers
It is possible for the “critical path” between the start milestone and the finish milestone to have several parallel branches of equivalent length. These can be described as “multiple critical paths.”
Depending on the details of the schedule model, it is not uncommon to have at least a few activities that are both concurrent and critical. That is, they comprise parallel branches of a common logic path that drives the completion of the project. For example, the construction schedule for a residential building may include one activity for “plumbing rough-ins” and another activity for “electrical rough-ins” (where “rough-in” is another term for “first fix” work.) The two activities have the same driving predecessors (e.g. structural framing) and driven successors (e.g. wall finishes), and they take the same amount of time to complete. If the wall finishes are on the driving path to project completion, then the two rough-in activities form parallel branches of the “Critical Path” for the project. Such parallel branches might be repeated for each floor of a multi-story building. In practice, instances of parallelism/concurrency that comprise only a few activities like those described here seem rarely, if ever, to be identified as “multiple critical paths.” This is because a) the parallel activities are seen as closely related; and b)traditional methods of identifying and depicting critical activities do not differentiate between the associated logic paths, typically sorting by dates and filtering primarily on the software’s “critical” flag and/or Total Slack/Float.
Multiple critical paths are also created by efforts to accelerate the project completion, such as crashing or fast-tracking exercises, after the initial development of the schedule. For example, a large scale construction project must be accelerated by 40 days to meet contract commitments. The critical path of the initial project schedule runs through building construction, while underground utility development activities possess 30 days of Total Float.
Additional resources (and costs) may be applied to compress the building construction activities by 30 days and yield a corresponding acceleration of the project completion. At that point, the building construction and utility development activities must both be compressed by an additional 10 days (and at additional cost) to obtain the necessary 40-day acceleration of the project. After the exercise, the building construction and utility development activities are equally driving the project completion, and the schedule possesses two critical paths.
Further crashing or fast-tracking exercises may add more critical paths. The associated activities all possess the same total float/slack, and all are marked as “critical.” Differentiating between the various paths requires a method for separately tracing and coding driving logic, either analytically or by visual inspection. Such methods are explored later in this article.
In practice, multiple critical paths are also created during project execution, as float is completely consumed by unplanned delays in activities that were previously non-critical. Exploration of such delays will have to wait for another article. (Notions of the “Critical Path” are sometimes suspended in the late stages of major/mega projects, as virtually every incomplete activity may delay the ultimate completion of the project and is therefore “Critical”.)
Ultimately, all three of these instances of multiple critical paths can be traced to a certain simplification (or presumed simplification) of resource utilization in the project. In the first example, plumbing and electrical crews are presumed to have exactly the same productivity, resulting in the same duration for both activities. In fact, one of the crews may be capable of completing the work sooner but paces its effort in order to match the scheduled duration. In the second example, it is presumed that the building construction activities and utility development activities may be compressed by exactly 40 days and 10 days, respectively. In fact, optimum resource usage often follows a step-wise rather than linear (or even continuous) function; consistent with adding discrete crews or substituting higher-capacity equipment. Thus, the building construction may be optimally compressed by 35 days or 45 days (not 40 days), and the utility development may be optimally compressed by 9 days or 12 days (not 10 days). As a result, the project completion is optimally accelerated by 42 days (not 40 days), with the critical path being governed by utility development. The crashed building construction activities get 3 days of float.
When taken to this level of detail, a single critical path may indeed be re-established. Depending on the needs of the project, such detail may or may not be justified in light of various project uncertainties and the increased management effort involved.
Multiple Delivery Milestones
When a project involves the parallel or interim delivery of multiple scopes of work, each delivery may be construed to have its own “critical path” – i.e. the sequence of activities and relationships determining the delivery date for the particular scope of work. More importantly, any particular activity in the project schedule may be expected to participate in more than one of these multiple critical paths. Examples include design, setup, or testing activities that may be common to several deliveries.
Here is a simple example project comprising six “phases” of inter-related tasks in a Microsoft Project (MSP) schedule.
MSP identifies the CP for the project on the basis of Total Slack (TS<=0), and it colors the associated bars red. The table includes six columns identifying the critical/driving tasks for each of the phase-completion milestones (CP1 through CP6). For example, the critical/driving tasks for the Phase 1 Completion (ID #6) are flagged “yes” and highlighted yellow in the CP1 column. This path comprises the following sequence, which is easily verified by inspection: 1->13->14->5->6. Interestingly, the first Phase 1 task — ID #3 – “1A” — is marked “critical” for the overall project but is not critical for the Phase 1 Completion. A delay of this task would delay the project but would not immediately delay Phase 1. Coincidentally, since Phase 6 is the last phase to finish, its critical path (column CP6) corresponds to the critical path for the overall project – i.e. the red bars and TS=0.
The project is scheduled identically using Oracle Primavera P6 (P6).
As a result of the modest number of inter-phase relationships, the critical/driving paths for five of the six phases include activities from other phases. The exception, Phase 3, is essentially self-contained, although several of its tasks are also driving the completion of Phase 1.
So, how are these six different critical paths identified?
Identifying Multiple Critical Paths
There is in fact no way to simultaneously define the critical paths to each of the six phase completion milestones of the example project using Total Slack alone without manipulation. The paths must be identified individually and directly reported (in the manner of BPC Logic Filter) or manually marked.
Deadlines / Late Constraints
It is common to apply deadlines (or late constraints in P6) to key completion milestones that are contractually defined – typically with financial consequences for delay. Deadlines have the potential to reduce Total Slack/Float, which MSP and P6 use to identify Critical tasks. When deadlines are applied to tasks and milestones that – together with their predecessor chains – are logically and organizationally separate, then Total Slack/Float can provide a reasonable indication of the driving path to each “deadlined” task or milestone. This is a rare circumstance. With only a single deadline applied, the Total Slack/Float for any task will be potentially influenced by the Project Completion date and by the deadline date, with the more “urgent” of the two forcing a lower Total Slack/Float value. With more deadlines and intersecting logic paths, the issue is multiplied.
In the present example, each phase completion task has been given a deadline corresponding to its current finish date. Consequently, all except two tasks in the entire project are given a Total Slack value of Zero and marked “Critical.” The delay of any of these tasks is certain to violate at least one of the deadlines, although the deadlines to be violated are not obvious without the table. The actual driving path to each phase completion remains obscured. Although not completely reliable for defining individual driving paths, deadlines (and late constraints in P6) remain useful for flagging those tasks whose delay could affect (or already have affected) a contractually significant milestone.
If the project already includes multiple deadlines (for contractually significant milestones), then the critical/driving path for each milestone can be identified (one at a time) by forcing the milestone onto the MOST critical path, i.e.:
Temporarily accelerating the deadline date to the point that it becomes the most “urgent” deadline in the schedule network for all affected tasks. The Total Slack of the milestone and its driving predecessors is then easily distinguished from that of other tasks; and then
Marking the tasks identified by the lowest Total Slack value. Here, the deadline for the Phase 1 completion was accelerated by 9 work days, resulting in a Total Slack of -9 days for the completion task and its driving predecessors. The CP1 custom flag field could then be marked accordingly.
As shown here, a nearly-identical process (using an accelerated Finish-on-or-before constraint) can be used for a P6 project schedule with multiple late constraints.
A similar approach can be used by adding a deadline or late constraint (in P6) if the project has none.
In fact this is the “Constraint Method” that was recognized by the Defense Contract Management Agency (DCMA) as the only valid method for defining a program critical path as part of its 14-point schedule assessment.
As shown above, using deadlines or constraints to generate negative slack/float is an effective way to identify multiple critical paths. In Microsoft Project, however, these paths are difficult to distinguish on the Gantt charts because their red bars are not uniquely differentiated from others. Both MSP and P6 can set the “Critical” flag – resulting in the red bars – for any tasks whose Total Slack/Float is below a certain user-specified threshold. Unlike P6, MSP does not permit this threshold to be less than zero. Consequently, all tasks with negative Total Slack – even those which are not driving anything – are flagged as “Critical” and given the red bar in MSP.
An alternate approach to overcome this limitation for presentation purposes uses a “super-long trailing dummy” task. In this approach, all deadlines and late constraints in the project must be removed (at least temporarily). Then a “trailing dummy” task is assigned as a successor to the first key completion milestone, forcing the milestone onto the critical path. The duration of the trailing dummy must be long enough to extend the completion of the project, creating positive Total Slack for all tasks that are not part of the dummy’s driving predecessor chain. As shown in the following two figures, adding the 100-day Trailing Dummy task as a successor to the Phase 1 completion milestone effectively creates a new critical path for the project – one which is easily recognized as the Phase 1 critical path on the Gantt chart. Moving the trailing dummy successor from phase to phase – one at a time – reveals the unique critical path for each phase.
So far all of these approaches rely on temporary manipulation of Total Slack or Total Float, and it is important that such temporary changes be reversed prior to sharing or distributing schedule files. Obviously, these approaches can be labor-intensive and error-prone, making them impractical when the schedule status (and logic) is in flux – as during regular weekly/monthly updating. Even when the complications of multiple deadlines or late constraints are removed, Total Slack also becomes unreliable as an indicator of critical/driving relationships whenever multiple task/resource calendars or resource leveling are applied.
Driving Logic Tracing
The “Longest Path” algorithm in P6 defines the project’s Critical Path by automatically tracing driving logic backward from project completion. This avoids the complications that late constraints and multiple calendars introduce to the interpretation of Total Float, and it is the preferred calculation method when these factors exist. (It is unfortunate that P6 seems to be the only mainstream project scheduling tool to implement it.) Since the algorithm always begins the backward trace with the project completion activity, using it with the trailing dummy method above is useful for identifying multiple critical paths on the basis of driving logic. Driving logic can also be traced in other ways.
Driving Path Trace / Filter (P6)
P6 automatically identifies driving and non-driving relationships in the task details. It also allows users to automatically add activities to existing filters by simply selecting them. The combination of these two features allows users to manually construct a filtered view of the critical path to any completion milestone by simply clicking backward through the driving relationships.
The first step is to construct a custom filter to isolate the completion milestone of interest. Here we are focusing on the Phase 1 completion milestone.
Next, the chain of driving activities (i.e. the driving path) is added to the view by stepping backward through the driving relationships using the “GoTo” button in the predecessors pane. Here is the view after taking the first backward step.
Here is the view after tracing the network all the way back to the first driving relationship. This is the driving (i.e. “critical”) path for the Phase 1 completion milestone.
This click-tracing technique can become tedious when the project schedule is complex and there are numerous branches to the logic paths. For example, if an activity along the driving path has two driving predecessors, then the analyst must make a note to return to the second one after the first is fully explored. In addition, P6 routinely marks all links to Level-of-Effort (LOE) activities and links to ALAP-constrained predecessors as driving. The LOE activities must be ignored during the trace, and the ALAP constrained activities need to be evaluated separately. Finally, near-driving logic paths can be identified by the “relationship free float” field in the predecessors table, but isolating driving and near-driving paths involves creating and marking a number of new activity fields.
Fortunately, P6 includes an advanced scheduling option for calculating multiple float paths, which I’ve previously written about here. This option effectively automates the click-tracing technique.
Combined with a view/layout/filter that depicts the “Float Path” field, this option provides a robust and repeatable method for defining the driving and near driving paths to each phase completion milestone in a P6 schedule. (The “free float” option must be used.) Float Path number 1 identifies the (first) driving path. Higher Float Path numbers identify any parallel driving paths and near-driving paths. Like the click-tracing technique, Multiple-Float Path analysis works without modifying the schedule network, so reversing temporary changes is not a concern prior to sharing the schedule. Neither of these techniques affects P6’s assignment of the “critical” flag, however, so red-bars on the Gantt charts are not meaningful.
Task Path (MSP)
If you are using a recent version of Microsoft Project (2013+), then the driving path to each phase completion milestone can be visually identified and highlighted on the bar chart using the “Task Path” function with the “driving predecessors” switch selected. (Task ID6 is selected, and the tasks in its “Driving Predecessors” Task Path are highlighted orange.) The user can then manually enter a code (like the custom flag fields shown above) to mark the highlighted tasks. It is fairly straightforward to automate the generation of a task path filter using a macro, and some macro snippets have been published. (E.g. here.) Unfortunately, Task Path’s “Driving Predecessors” are not reliable for most complex projects (e.g. when non-Finish-to-Start links, in-progress tasks, or manually-scheduled tasks are present.) Those issues are discussed in my other blog entry here.
BPC Logic Filter (an MSP add-in) was developed in part to offset the absence of the Longest Path and multiple float path analysis in MSP. The chart below shows driving and near-driving paths to Phase 1 completion – depicted by altering the bar chart. The driving path is indicated by dark red bars with the zero indicating zero relative float. Consistent with the trailing-dummy results, the two non-driving tasks that are part of Phase 1 are depicted as having 2 days and 3 days, respectively, of float relative to the Phase 1 completion milestone. That is, they could slip 2 or 3 days before affecting the milestone. Similar depictions of the driving and near-driving paths for the other five phases, and for the project as a whole, are made possible with a few clicks. Since the schedule network is not manipulated in any way, Total Slack remains unchanged, and there is no need to reverse any analysis-based modifications prior to sharing the data file.
A Note About Open Ends
Some experts have suggested using open-ended logic along with a built-in software calculation setting to automatically mark multiple critical paths. The proposal is as follows:
Ensure that each of the Phase completion milestones is sequenced so that a) it is a logical successor to all the activities in the phase, and b) it has no successors.
Enable a calculation setting that forces activities with no successors to have zero total float. In P6, this setting is the “Make open-ended activities critical” schedule option. In MSP, the setting is the “Calculate multiple critical paths” advanced calculation option.
Effectively, this is the same as assigning a deadline to each completion milestone at a date that exactly equals its early finish date – an example of which was already explored above. It is subject to the same drawbacks of that approach, but with none of the advantages. I.e. If there are intersecting logic paths between phases (as shown in the example), then which activities are driving which milestone cannot be determined based on total float alone. One merely sees a bunch of zero-float activities in all the phases. Moreover, the basic technique of accelerating the milestone’s deadline/late-constraint date to reveal its driving predecessor path (through negative float) is not available when no deadline or late constraints have been applied. Between this and other failings, I just don’t see any advantages to this open-ends approach.
A project schedule can possess multiple critical paths for one of two primary reasons:
There is a single key completion milestone at the end of the project, and multiple, concurrent, parallel driving paths to that milestone exist. In this case, the multiple critical paths often reflect a schedule model that is simpler than the actual execution of the project. Correctly accounting for productivity of the assigned resources may remove the apparent concurrency and restore the single Critical Path, but the increased overhead of developing and managing a more detailed schedule would need to be justified.
The project possesses multiple key completion milestones, each with its own legitimate driving/critical path. In this case, the correct identification and status reporting for the multiple Critical Paths is often beyond the capabilities of the scheduling software. Then specialized logic tracing techniques are required.
These factors can also be combined in some project schedules.
In the presence of Deadlines, Constraints, variable Calendars, and resource leveling, Total Slack becomes unreliable as an indicator of the Critical Path (or of nearness to the Critical Path). In addition, many projects include Key Completion Milestones that occur long before the final scheduled activity of the project, so a Longest-Path approach doesn’t apply. For these projects, I use the Task Logic Tracer to find the Driving Path and Near-Driving Paths of each Key Completion Milestone.
In the presence of Deadlines, Constraints, variable Calendars, and resource leveling, Total Slack becomes unreliable as an indicator of the Critical Path (or of nearness to the Critical Path). For projects where the project completion is designated by the last task in the schedule, I use the Near Longest Path Filter to keep an eye on next week’s concerns….
In the presence of Deadlines, Constraints, variable Calendars, and resource leveling, Total Slack becomes unreliable as an indicator of the Critical Path. For projects where the project completion is designated by the last task in the schedule, I use the Longest Path Filter to identify the Critical Path….