Multiple Critical Paths – Revisited with BPC Logic Filter

While writing an article on Multiple Critical Paths a few years ago, I knew that BPC Logic Filter (our add-in for Microsoft Project) could easily identify multiple critical paths (and near-critical paths) in any project schedule, but we needed to analyze key milestones one at a time.  In most cases I think that’s still the best approach to understanding the factors that drive (and may delay) key project deliverables.  Nevertheless, the latest build of the software (1.5.5.13) includes a new setting that facilitates simultaneous analysis and display of the critical paths to multiple milestones in a project or program schedule.

The setting – Group Results by Selected Task – is found on the Tracing Preferences tab of the software settings, and we enabled it as part of the initial distribution of the build.  We hope that at least a couple users have been pleasantly surprised.

This short article provides a couple illustrations of the feature in action.

First, here is the simple example project from the previous article.  The project comprises six “phases” of inter-related tasks in a Microsoft Project schedule, now displayed using MSP 2016 rather than MSP 2010 as before.  The project has no deadlines or constraints, and MSP designates critical tasks (red bars in the chart) based on total slack (TS<=0).

In the previous article, I illustrated – using MSP and Oracle P6 – several generic techniques to identify the unique critical path for each of the six phase completion milestone in the project.  I also used BPC Logic Filter to graphically illustrate the critical- and near-critical paths for the Phase 1 completion milestone, displayed within the context of the overall project (and repeated here in MSP 2016).

Going back to the original project, we can now run the task logic tracer with all six phase-completion milestones selected.  The Standard Edition of BPC Logic Filter then allows us to select driving-path predecessors and to re-sort the results to show logical branches (i.e. paths).

The result is a grouped view displaying the driving predecessor path to each of the six phase-completion milestones.

Due to intersecting logic, some tasks are on the driving/critical path for more than one completion milestone, but the Gantt chart only allows them to be displayed once.  The task logic tracer displays such tasks together with the driven milestone that is encountered first in the original task selection.  Thus, while tasks 1A, 4A, and 4B are critical for the overall project and for the phase-6 completion, they are displayed with the phase 4 completion milestone – for which they are also driving/critical – because that milestone was encountered first in the user’s selection.  As a consequence, re-sorting the tasks in the overall project can alter the apparent driving paths for key delivery milestones when intersecting logic is present.

The same caveat applies when considering near-driving/critical paths for multiple key milestones in the same project, along with an added condition that a non-driving task will be included with whichever key milestone it is nearest to driving.  To illustrate, we have re-run the previous analysis while considering path relative float, and we’ve decided to re-color bars to clarify results.

The resulting output is like the earlier one, but now including annotated bars and tasks whose path relative float is above zero (i.e. non-driving).

From the earlier article, we already know that task 1A is a non-driving path predecessor of the phase-1 completion milestone, 2 days from driving that milestone.  It is nearer to driving (and is in fact driving) the phase-4 completion, however, so that is where it is shown.

As we’ve seen, it can be difficult to differentiate the critical paths to multiple completion milestones in a complex project with lots of intersecting logic paths.  When a project or program includes multiple completion phases that are not closely related, however, the output is straightforward.  A good example of this case occurs when multiple unrelated subprojects are combined into a linked master project for reporting purposes.  Here I’ve used the New Window dialog to temporarily combine three simple projects into a temporary master.  Then I have applied a filter to show only the key completion milestones for the three subprojects.  Finally. I’ve run the task logic tracer with all the visible tasks selected.  A Pro Edition of the tool is required to analyze linked subprojects, and I’ve limited the relative float analysis to keep the resulting output small.

The resulting layout clearly depicts the critical/driving- and near-critical/driving paths for each subproject completion milestone.  As usual – for BPC Logic Filter – the definitions of critical/driving logic paths do not rely on the total slack, which is not reliable in many modern project schedules.

 

 

Driving Logic in Backward Scheduled Projects (Microsoft Project)

Microsoft Project’s backward scheduling mode includes numerous traps for novices and professionals alike.  Competent project schedulers avoid it.

In MSP, a backward scheduling mode – sometimes called backward planning, reverse scheduling, or reverse planning – can be invoked by scheduling the project from its finish rather than its start.  In the traditional language of critical path method (CPM) scheduling, it’s most simply described as a late-dates schedule.  Backward planning is useful in several non-standard methodologies, including critical chain project management and the pull planning aspects of the Last Planner System.

The Mechanics of Backward Scheduling

When specified for the active project, this mode essentially does the following:

  1. Sets the default constraint type for all new tasks to as late as possible (ALAP).
  2. Re-sets the constraint type to ALAP for all existing Summary tasks.
  3. [Users choosing this mode in the middle of schedule development must manually re-set the constraint type to ALAP for all existing non-Summary tasks.]
  4. Automatically sets no later than constraints when dates are manually entered into the start or finish fields of tasks.  [Such date entry in forward scheduling mode leads to “no earlier than” constraints, so users choosing this mode in the middle of schedule development should manually review, validate, and potentially re-set any previously-entered date constraints.]
  5. Performs the network scheduling calculations in reverse order, with the reflection point occurring at the project start rather than the project finish.  I.e. the project start date (not the finish, which is user-input) is determined by the logic.
  6. Sets the start and finish of automatically-scheduled tasks to their late dates rather than their early dates.  (This is the most important part.)
  7. Finally, the resource leveling engine resolves resource over-allocations by accelerating higher priority tasks from late dates rather than delaying lower priority tasks from early dates.  Thus, entries in the leveling delay field are negative.  This behavior creates a minor complication regarding use of priority = 1000.  Just as in forward scheduling, a task with priority=1000 is always exempted from any leveling action.  In backward scheduling, this means that priority values of 1000 and 0 are essentially equivalent when considered in the leveling decisions.  The highest effective priority for controlling leveling behavior then becomes 999, not 1000.

Logic relationships used in backward scheduling still have exactly the same meanings that they do in forward scheduling.  A finish-to-start (FS) relationship still means that the two tasks are logically connected such that the successor may not start before the predecessor finishes, and the rarely-applicable start-to-finish (SF) relationship still indicates that it is impossible for the successor to finish before the predecessor starts.  Some users seem to think that backward scheduling involves reversal of these two relationships in particular, but that’s not consistent with the rest of the backward scheduling mode.  Unfortunately, mixing of the two approaches seems to continue, though this typically amounts to invalid date manipulation in my view.

In normal (i.e. forward) scheduling, a task with an ALAP constraint has the dubious distinction of corrupting its entire chain of successors – driving all of them to the critical path.  There are very few legitimate applications for this constraint.  In backward scheduling, the as soon as possible (ASAP) constraint plays a similar role, corrupting its chain of predecessors.  It needs to be avoided in backward scheduling.

When to Use Backward Scheduling

I’ve never used backward scheduling in a real project.  Others have recommended its use to determine the desired start date of a project when the desired completion date is already known.  It also seems consistent, when tasks are suitably buffered, with aspects of critical chain project management that require work to be scheduled as late as possible.

Ultimately, backward scheduling rests on the presumption that tasks can be accelerated (i.e. moved to the left on the bar chart) indefinitely as needed to meet the fixed end date for the project.  Thus, a task whose duration is extended can simply be re-scheduled to start sooner than previously planned, with its predecessors also being accelerated.  Similarly, a higher priority task can be started (and finished) earlier to avoid resource conflicts with a lower-priority task that demands the same resources.  The problem with these presumptions is that time invariably marches forward, and as scheduled dates for incomplete work are overtaken by the vertical time-now line on the bar chart there is no chance for recovery.  The backward scheduling method is pointless if the latest allowable project start date has already been passed – e.g. the project is in progress.

Backward scheduling seems to be of primary value in determining the latest responsible date to start a project (or project segment) while still meeting the desired completion date.  After that latest responsible start date is determined, the project must be converted from backward-scheduled to forward-scheduled mode – manually reviewing and revising the key parameters of each task – if it is to be used for updating and forecasting during project execution.  The original question – i.e. what is the latest responsible project start date? – is just as easily answered by manipulating and examining the late dates of the forward scheduled project.  Thus, for a competent project scheduler, the use of backward scheduling seems largely to be an unproductive diversion.

Driving Logic Analysis

When the logic network is well constructed – and complicating factors like multiple calendars, (early) constraints, and resource leveling are avoided – then the critical path may be reasonably identified by total slack = 0.  Other methods of driving logic analysis must be modified, however.

Under backward scheduling, any slack/float of a task exists on the side towards its predecessors, i.e. to its left on a bar chart.  A driving relationship exists when a successor prevents a predecessor from being scheduled any later than it is.  This means that there are driving successors and driven predecessors.  Consequently, the Longest Path in a backward scheduled project is the driving (successor) path from the project’s start.

MSP includes two built-in methods for reviewing and analyzing driving logic: the Task Inspector pane and the task paths bar styles.  As I wrote in this article a few years ago, I’ve found these tools to be unreliable in complex real-world project schedules.   Under backward scheduling, they are essentially useless and/or misleading.

To start with, Task Inspector simply doesn’t work with backward scheduling.  Opening TI on a backward scheduled project yields the following message:  This project is set to Schedule from Finish.  We are unable to provide scheduling information.

Also under backward scheduling, the Driving Predecessors and Driven Successors bar styles are still derived from Early Dates, as they are in Forward Scheduling.  This makes them essentially useless for assessing the controlling logic of the displayed (late-dates) schedule.  Consider the example below, where all four Task Path bar styles have been imposed, and Task 11 – A2 Structures – is selected.  (The automatic “Slack” bar style is also imposed, but it is invisible since Free Slack – formally defined by Early dates alone – is uniformly zero.)

The selected task is in fact driving/controlling the displayed dates of both of its predecessors (LRF=0), but only one of them displays the correct bar style (the one that, with ERF=0, was the driving predecessor during the forward pass).  Of Task 11’s four successors, only the first (Task 13 – A2 Electrical) is directly driving/controlling Task 11’s schedule, with LRF=0.  The tasks for two of the remaining three successor relationships are incorrectly highlighted, while the third (Task 14) is correctly highlighted only because it is driving/controlling Task 13 – a case of redundant logic.  (All four successors were driven successors during the forward pass.)  Thus in a backward scheduled project, the Task Path bar styles for Driving and Driven dependencies are meaningful (or “correct”) ONLY along the Longest/Critical path of the project, where Early dates and Late dates coincide.  The relationships along that path are driving in both directions; i.e. they are bi-directional driving relationships.

BPC Logic Filter – my company’s Add-In for logic analysis of MSP schedules – identifies driving logic based on relationship free float, which we often call “relative float.”  In BPC Logic Filter, the Longest Path and near-longest paths of simple, backward-scheduled projects can be found using the Task Logic Tracer, starting from the project start milestone and using appropriate settings (i.e. driving relationships in successor direction).  As illustrated in the example project, this is fairly trivial since the results are 100% aligned with Total Slack.

Other driving logic paths (not on the Critical Path) are not so trivial but are easily addressed using BPC Logic Filter, provided that the impact of multiple calendars is minimal.

Precision analysis of more complex, backward-scheduled projects requires some modest modifications to the algorithms.  In particular, several variations of late-relationship-float need to be computed, and these have been added to the development roadmap for the software.

[Apr’20 Revision.  I’ve updated the two figures to incorporate the early-date relative float (ERF) and late-date relative float (LRF) columns in the logic inspector windows.  Calculation of the latter was only added to recent builds of BPC Logic Filter.  These windows also flag the forward-driving (yellow), backward-driving (rose), and bi-directional-driving (red/yellow) relationships.]

Avoid Out-of-Sequence Progress in Microsoft Project 2010-2016

[For the corresponding video presentation, see Construction CPM Conference 2020 – Avoid Out of Sequence Progress in Microsoft Project.]

Recording Actual dates that violate existing schedule logic can cause conflicts in Microsoft Project’s internal schedule calculations. The resulting Total Slack values and Critical task flags can be incorrect and misleading.  These issues are aggravated by recent (e.g. MSP 2016) versions of the software, and users are advised to minimize out-of-sequence progress.

Recording of actual progress in a logic-driven project schedule can be problematic.  As the “Actual” dates override or otherwise constrain the computed dates, the customary definitions of Float or Slack – and their resulting impacts on the “Critical” task flag in Microsoft Project (MSP) – no longer apply.  While I hope to undertake a general review of progress updating issues in a future article, this one has a special focus on out-of-sequence progress for two primary reasons:

  1. In all modern versions of Microsoft Project (e.g. ~MSP 2007+), the Total Slack values of “Critical” tasks with out-of-sequence successors can be altered unexpectedly.
  2. In MSP 2016 (and possibly beginning with MSP 2013), the Total Slack values of ALL tasks (not just “Critical” tasks) with out-of-sequence progress among their successors can be altered unexpectedly. As a result, many tasks can be shown incorrectly as “Critical” in MSP 2016 when they are not “Critical” in earlier versions.

What is Out-of-Sequence Progress

Out-of-sequence progress exists when actual progress is recorded (via, e.g. Actual Start, Actual Duration, Actual Work, %Complete, etc.) at times when the logical constraints of the schedule would normally preclude it.  For example:

  • An Actual Start is recorded for a task (the out-of-sequence, or OOS, task) whose Finish-to-Start predecessor has not finished. I.e. the Actual Start precedes the Early Start;
  • An Actual Start is recorded for an OOS task whose Start-to-Start predecessor has not started. Again, the Actual Start precedes the Early Start;
  • An Actual Finish is recorded for an OOS task whose Finish-to-Finish predecessor has not finished. Here the Actual Finish precedes the Early Finish.
  • An Actual Start is recorded for an OOS task whose Finish-to-Finish predecessor subsequently delays the completion of the work (on the OOS task).  Again the Actual Start precedes the Early Start that would have existed in its absence.

In all cases there is a presumption that the recorded actual progress is more correct than the (theoretical) schedule model, so Early and Late dates are routinely overwritten by Actual dates during the schedule calculations.  When the actual progress occurs out of sequence, however, computing the Late dates (and slack values) of incomplete predecessors (during the “Backward Pass” calculations) is complicated by logical conflicts.  The software typically resolves these conflicts in a way that satisfies the needs of most users.

Completed, Out-of-Sequence Tasks

The issues discussed here are of primary concern in those cases where a task has started out of sequence and a) it remains incomplete; and b) the violated predecessor relationships remain unsatisfied (e.g. the FS predecessor remains incomplete.)  If either the OOS task or its unfinished predecessor task become complete, then they are treated like other completed tasks in Microsoft Project.  That is, they can influence the Early dates of their successors but have no impact on the Late dates of their predecessors.  As a result of this latter condition, an incomplete task whose sole successor has been completed out of sequence becomes effectively open-ended, i.e. without successors.  Under default conditions (i.e. “calculate multiple critical paths” NOT checked), the task’s Late Finish is set equal to the project’s Early Finish Date, and a high value of Total Slack is computed.

Obviously, this can have a major impact on the apparent Critical Path of a project.  In the example below (which is further described in later paragraphs), tasks CP2 and SP2 have both been completed out-of-sequence, and at a duration of only 20% of their baseline durations.  The overall project has been shortened, but the Critical Path has been truncated at CP3.  CP1 is no longer “Critical” because, in effect, it no longer has any successors.  It appears necessary to add a new FS relationship from CP1 to CP3 (and equivalently from SP1 to SP3) to re-establish the logic chain that has been broken by the completed, out-of-sequence tasks.

In Oracle Primavera P6, by contrast, the Retained Logic schedule setting automatically adjusts for activities completed out of sequence.  Thus, the need to finish CP1 before proceeding with CP3 is already included.  The result is a (presumably more realistic) 2-day delay in completion compared to the MSP result.

In-Progress, Out-of-Sequence Tasks

Key issues arise when the out-of-sequence task and its predecessor are both incomplete.  Because this behavior is sometimes different for MSP 2016 than it is for MSP 2010, we’ll look at both versions for the remaining examples.

For exploring the behavior of in-progress, out-of-sequence tasks, we examine the simple project schedule below.  The schedule is comprised of a single start milestone, a single finish milestone, a “Critical Path” string of four tasks, and a “Slack Path” string of four tasks.  The “Slack Path” is two days shorter than the “Critical Path,” with the last two tasks each having a shorter duration.  There is an unachievable Deadline applied to the finish milestone, and this creates negative Total Slack on the Critical Path.  Thus, the Critical Path tasks all have TS=-1d, and the Slack Path tasks have TS=1d.

With no progress, the project is scheduled identically in both versions of MSP.  Notably, P6 also starts with the same schedule dates.

Now let’s examine what happens when we record an out-of-sequence Actual Start on some future task.  In the example, the last task in each string (CP4 and SP4) is given an Actual Start that is one day earlier than its predecessors allow.  To keep things simple, no progress beyond the actual starts are recorded (i.e. %Complete = 0%.)  I’ve kept the “Split in-progress tasks” scheduling option checked (per default), so re-scheduling the project creates an initial split in tasks CP4 and SP4 and delays their remaining parts to satisfy the predecessor relationships.  As a result, all the tasks keep the same finish dates as before, and the project finishes on the same date as before, one day after the Deadline.

Although their start and finish dates have not changed, the logic-related information of the predecessors of the OOS tasks have been altered substantially.

  1. In both MSP 2010 and MSP 2016:
  • The Total Slack values of the Critical Path tasks that precede the OOS task (i.e. tasks CP1, CP2, and CP3) are all changed from TS=-1d to TS=0d.
  • This behavior is not justified: if the tasks are all executed according to the scheduled dates, the project will still finish one day late. The tasks should still have TS=-1d.
  • This is in fact a general result (see also Comment 1): (Presumably for MSP 2007 through MSP 365), any super-critical (TS < 0) task with an out-of-sequence, in-progress task in its successor chain will automatically have its Total Slack re-set to zero. Thus, out-of-sequence progress can cause a task with 60 days of negative Total Slack to appear much less Critical than it is.
  • This behavior can present a problem for project managers operating in a negative-slack (i.e. behind-schedule) regime, where schedule-recovery efforts are typically prioritized based on Total Slack values. Entering a single OOS Actual Start value (whether correct or not), can substantially alter the overall schedule recovery picture.
  • It seems most MSP users pay no attention to Total Slack values beyond the application of the “Critical” flag, and the observed behavior doesn’t change that. Consequently, for most users up through MSP 2010, out-of-sequence progress appears to have no substantial impact on the “Critical Path.”
  1. In MSP 2016, in addition to the prior behavior:
    • In the example, the Total Slack values of the Slack Path tasks that precede the OOS task (i.e. tasks SP1, SP2, and SP3) are all changed from TS=1d to TS=0d.
    • This behavior is also not justified. The tasks could all be delayed one day from their scheduled dates without compromising the project’s completion Deadline.  The tasks should still have TS=1d.
    • This is also a general result (see also Comment 1): (presumably for MSP 365 and maybe MSP 2013), ANY task (Critical or non-Critical) with an out-of-sequence, in-progress task in its successor chain will automatically have its Total Slack re-set to zero. Consequently, it will automatically and unavoidably be flagged as a Critical task.
    • For general MSP users after MSP 2010, therefore, out-of-sequence progress can have a substantial, even major, impact. In particular, tasks that are actually far from the Critical Path may be incorrectly flagged as Critical.
  2. The results in Oracle P6, however, appear more realistic.  The Late dates and corresponding Float values are unchanged from the initial schedule.

Below I’ve shown another perhaps more realistic example of the same simple project.  There has been a simple progress update on the Status Date of 1Oct’18, four working days into the project.  As of that date (the first Monday in the project), tasks CP1 and SP1 are still in-progress and have fallen one day behind the original plan.  Their successors (CP2 and SP2) have been allowed to start early, however, with each recording one day of actual progress.

Similar logical results are observed.  Task CP1 – the Critical predecessor of the Critical OOS task CP2 – now has TS=0d instead of TS=-1d in both software versions, and its Critical flag remains unchanged.

In MSP 2016 only, task SP1 – the non-critical predecessor of the non-critical OOS task SP2 – now also has TS=0d and is flagged as Critical.  This is incorrect.

As before, however, the late dates and Float values in the P6 version of the schedule are aligned with expectations.

Out-of-Sequence Progress and Task Path Driving Predecessors in MSP 2016

The “Task Path” bar styles provide useful methods for identifying related tasks, including the Driving Predecessors path, for any selected task in MSP 2013+.  The Driving Predecessors Task Path is particularly useful for confirming the Critical Path of a project when Total Slack is complicated by other factors.  Unfortunately, the method is not successful when out-of-sequence progress is encountered.  As shown in the figure below – repeating the two previous examples in MSP 2016 – the Driving Predecessors Task Path (orange-colored bars) is terminated when an Actual Start is reached on the backward (right-to-left) pass.  Thus, driving Task Path functionality is not compatible with out-of-sequence progress.

[The Task Path functionality is equally incompatible with in-progress schedules that involve Finish-to-Finish relationships among overlapping tasks, even if none of the progress occurs out of sequence.  Any Actual Start value terminates the progression of the associated bar formatting flag.]

Out-of-Sequence Progress and BPC Logic Filter

The impacts of out-of-sequence progress on the Total Slack of some tasks are the result of specific decisions in MSP’s backward-pass algorithms for computing Late dates and Total Slack.  Obviously, the algorithm has been tweaked between MSP 2010 and MSP 2016 leading to the even more undesired results.

BPC Logic Filter – my company’s MSP add-in for logic analysis – generally ignores MSP’s Deadlines, Late dates, Actual dates, and Total Slack values.  Instead, it performs separate backward and forward traces to determine driving logic paths and relative float values.  The latter, like Free Float, can never be negative.  Thus, when BPC Logic Filter encounters out-of-sequence progress during a trace, zero relative float is applied, and a driving relationship is inferred.  As shown in the examples below, this approach results in the correct identification of driving and near-driving paths to project completion, even when out-of-sequence progress is encountered.  [In the bar charts, the path relative float is listed to the right of each bar.  A zero-value represents the driving path with the bar characteristically colored (maroon), positive values and associated bar colors indicate the number of days away from driving the project completion.  

BPC Logic Filter also includes a Project Logic Checker to identify logic issues in Microsoft Project tasks.  Like many such tools, it automatically flags OOS tasks, along with their immediate predecessors, for correction. (Beginning with release, 1.5.5.5, the tool also differentiates the important categories of OOS tasks.  The “-I” suffix in the bar chart below identifies out-of-sequence conditions where both predecessor and successor of the violated relationship remain Incomplete; total slack in such cases cannot be trusted.)

In addition, the Logic Inspector in BPC Logic Filter automatically flags tasks with out-of-sequence progress as well as all the relationships that are violated by such progress.  Here it is shown that the out-of-sequence progress on task SP2 violates its predecessor relationship with SP1.  The late dates and total slack of the predecessor (SP1) are incorrect.

Out-of-Sequence Progress in the Real World

This examination was prompted by an associate who, after a recent “upgrade” from MSP 2007 to MSP 2016, encountered numerous unexplained changes in the “Critical Path” during project updating.  As it turned out, the updated schedule contained extensive out-of-sequence progress that explained the observed behavior.  The out-of-sequence progress was the result of a schedule model that was ultimately invalid – a poor representation of the work, either as planned or as actually executed.  As is often the case in construction, this was aggravated by the persistent executive schedule pressure that converts some technological restraints (e.g. don’t start interior finishes until the building roof and skin are closed up) from mandatory requirements into mere preferences.

A typical invalid schedule model involves the representation of multiple overlapping activities as an over-simplified Finish-to-Start string.  For example, the schedule below shows five sequentially-related, non-critical tasks, of roughly equivalent work content, that are scheduled to occur over a five-week period.  Although they are shown sequentially, it is in fact customary to execute these five tasks nearly concurrently, with each task commencing as soon as its predecessor’s progress allows – and thereafter suspending or pacing progress to match that of its predecessors.

[Some schedule purists would suggest that these tasks should be broken down into many small, repetitive work packages, all arranged with pure Finish-to-Start logic relationships.  The resulting schedule is extremely detailed and reflects a true logical plan for executing the work.  In my experience, however, such a detailed plan can often be riddled with preferential logic that is ultimately over-ruled by field decisions.  Out-of-sequence progress – in spades – is the inevitable result; the scheduling workload multiplies with no added value.

Another approach might involve five parallel tasks, each of five weeks duration, modeled with continuous relationships or at the very least with compound (joint SS + FF) relationships.  Unfortunately, neither relationship is supported by MSP.  The dummy-milestone approach that I use to mimic compound relationships in MSP seems fairly esoteric, and the common alternative – using solely SS or FF relationships – can be problematic.]

The scheduler here has chosen the most expedient route, a simplified Finish-to-Start string of five activities.  Four weekly updates of actual progress then lead to the progressed schedule shown at the bottom of the figure.  While its appearance is substantially changed compared to the original plan, the progressed schedule – in MSP 2010 – seems to correctly depict the status and slack of the multiple in-progress, out-of-sequence tasks.  Thus MSP 2010 seems largely indifferent to the consequences of invalid logic combined with out-of-sequence progress, as long as the affected tasks are non-critical.

As my associate discovered, however, MSP 2016 is far less tolerant of such practices.  When the progressed schedule is recalculated in MSP 2016, the in-progress predecessors of the in-progress, out-of-sequence tasks are shown as Critical (with TS=0).  This is incorrect, as the tasks could all slip by 15 days without delaying the project.

For comparison, here are the corresponding updates for the identical project schedule in Oracle P6 (R18.8.10).  The dates and Float values are identical to those for MSP 2010; they are correct for these non-critical tasks.  (Original and Actual Durations in P6 are handled differently than in MSP, so those are not comparable.)

Review and Recommendations

Out-of-sequence progress is an unwelcome but often unavoidable occurrence in projects with logic-driven schedules.  It happens when the actual project execution is allowed to deviate from the plan in a way that creates logical conflict in the automatic scheduling calculations.  It typically results from a combination of the following circumstances:

  1. The schedule plan includes logic relationships that are not technologically mandatory. I.e. there are a number of alternative methods available for sequencing a group of related activities, and only one (preferred) method is incorporated into the schedule.  In addition:
    • Procedural controls are inadequate to ensure that project execution conforms to the preferred logic sequence; or
    • Subsequent to initial schedule development, the preferred logic sequence is altered due to field conditions, resource limitations, or other factors.
  2. Subsequent to initial schedule development, technologically-mandatory logical constraints are allowed to be violated – typically under schedule performance pressure – and the resulting technical risks are accepted.
  3. The schedule is based on scope and logic definitions that are overly simplified compared to the actual or achievable plan of execution. E.g. simple Finish-to-Start relationships are used to represent overlapping, partly-concurrent activities.
  4. Subsequent to occurrence of any of the prior circumstances, the schedule logic is not revised appropriately.
  5. Schedule updates do not include regular review and correction of invalid logic in light of out-of-sequence progress.

In MSP, a task that is both started and completed out-of-sequence may cause the incomplete tasks in its predecessor and/or successor chains to become effectively open-ended, with impacts on Early and Late dates, Total Slack, and Critical task definition.  Consequently, the updated schedule may be invalid.

A task that is started out-of-sequence but remains in progress may cause substantial alterations to the Late dates, Total Slack, and Critical definitions of the incomplete tasks in its predecessor chains.  In particular:

  1. (In all modern MSP versions) Total Slack of behind-schedule tasks will change from negative to zero and in some cases turn positive. [See also comment 1.] Although the Critical flag won’t always change, any identification of driving and driven logic paths that is based on negative Total Slack will be incorrect.
  2. (In recent versions – e.g. MSP 2016) Total Slack of non-Critical tasks will change from positive to zero (with some exceptions), and each task will be incorrectly marked as Critical. [See also comment 1.]  Thus, the “Critical Path” and any other Slack-based identification of driving or driven logic paths will be incorrect.

These are pretty major consequences, yet their persistence suggests that they reflect as-designed behavior, not bugs.  It might even be suggested that the most recent “improvements” are intended to highlight the out-of-sequence progress – for correction of the associated logic.

In MSP, the only reliable way to avoid the negative consequences of out-of-sequence progress, in my opinion, is to avoid and/or minimize its occurrence.  Fundamentally, this means:

  1. Ensure that project schedules are based on sound consideration of the scope and logic of project execution.
  2. Ensure that procedural controls are put into place to a) validate, b) revise where necessary, and c) enforce the preferred sequence of activities.
  3. Where necessary, revise the schedule logic to reflect the actual/required sequence of execution.
  4. During regular progress updates, identify all out of sequence progress, and revise associated logic as appropriate to avoid the consequences noted above.

 

Understand the Impact of Calendars on Schedule Slack Calculation in Microsoft Project

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

  1. 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.
  2. 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.
  3. Relationship lags are computed according to the Project Calendar.
  4. Start Slack, Finish Slack, and Total Slack are computed using the Project Calendar.
  5. The default calendar for ProjDateAdd, ProjDateSub, and ProjDateDiff functions is the Project Calendar.*
  6. 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.
  7. 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

  1. 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.
  2. 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.
  3. Relationship lags are computed using the Project Calendar.
  4. Start Slack, Finish Slack, and Total Slack are computed using the Project Calendar.
  5. 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.*
  6. 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)

  1. 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.)
  2. 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.
  3. 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.
  4. Relationship lags are computed according to the Task Calendar of the successor task, if it has one, or the Project Calendar.
  5. Start Slack, Finish Slack, and Total Slack for each task are computed using the Task Calendar, if it has one, or the Project Calendar.
  6. 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.*
  7. 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.

D. Elapsed-Durations

  1. 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.)
  2. 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.)
  3. 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.
  4. 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.
  5. Non-elapsed relationship lags are computed according to the Task Calendar of the successor task, if it has one, or the Project Calendar.
  6. Start Slack, Finish Slack, and Total Slack for each elapsed-duration task are computed on the basis of elapsed time.
  7. For tasks with elapsed durations, the default calendar for ProjDateAdd, ProjDateSub, and ProjDateDiff functions used in custom Task fields is the 24-Hour Calendar.*
  8. 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:

  1. Each task is scheduled only during work time that is available in BOTH the Task Calendar and the applicable Resource Calendar for each assignment.
  2. 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.
  3. Relationship lags are computed according to the Task Calendar of the successor task, if it has one, or the Project Calendar.
  4. Start Slack, Finish Slack, and Total Slack are computed using the Task Calendar, if any, or the Project Calendar.
  5. 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.*
  6. 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.

 

Don’t Confuse Critical Tasks with Critical Paths in Project Schedules

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:

  1. A CPM project schedule is comprised of all the activities necessary to complete the project’s scope of work.
  2. 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.
  3. 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.
  4. 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.]
  5. 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.
  6. 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.”]
  7. 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.]
  8. 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:

  1. 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.
  2. 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.
  3. 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.

     

  4. 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.)  More generally,  when a driving logic path contains activities with different calendars, the interval between the early and late dates of one activity may contain more or less worktime than the corresponding intervals of its mates on the path.   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:

  1. 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.
  2. 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.
  3. 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.  Moreover, the description of (and objection to) a broken Critical Path rules out driving paths with discontinuities caused by early date constraints.

(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 early-date constraints, multiple calendars, multiple late-date 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:

  1. 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.
  2. 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.
  3. 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.
  4. “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.

Recap

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. A critical task/activity is best defined (in my opinion) as either:
    1. An activity that resides on the critical path; or
    2. An activity whose delay will lead to unacceptable delay of the project completion; or
    3. An activity whose delay will lead to unacceptable delay of some other constrained activity or milestone.
    4. In general, these conditions are mutually exclusive, and different activities within a single project schedule may satisfy one or more of them.
  7. 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.

 

Video – Find the Driving Path for Key Milestones in Microsoft Project using BPC Logic Filter

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.

Video – Using BPC Logic Filter to Analyze Resource-Leveled Critical Path

Here’s another video of BPC Logic Filter in action – this time revisiting the themes of  previous blog entry:  The Resource Critical Path

 

A Logic Tracing Example in Microsoft Project

This article uses BPC Logic Filter to present the progression of a single logic tracing example from a simple approach to a more focused analysis. [June 2020: revised for updated versions of MS Project and BPC Logic Filter.]

[If you came here looking for a simple logic tracing macro and are comfortable using Visual Basic for Applications (VBA), have a look at these two other entries: Macro for tracing filtering and sorting task paths in Microsoft Project and Simple Macro for Listing Driving Predecessor(s) in MS Project.  They may give you what you want as long as your project has simple logic.]

BPC Logic Filter is an excellent tool for defining and visualizing the Critical Path and Near-Critical Paths of a project when Total Slack proves inadequate – namely in the presence of constraints, variable calendars, and resource leveling.  As I’ve written elsewhere, however, the first version of BPC Logic Filter was prompted by a very different, though straightforward stakeholder request: “My people can’t see why they need to finish these tasks so soon. Isn’t there a report or something to show what other tasks (in other departments) are depending on them?”  In other words, couldn’t I group, filter, and sort tasks simply according to logical relationships – in addition to Work Breakdown Structure, responsible department, and other codes.  At its core, the resulting solution was a simple logic tracing routine for exploring, marking, and displaying logical relationships in an existing project schedule.

This article presents the progression of a single logic tracing example from a simple approach to a more focused analysis.

Consider the example of the small installation project presented below. There is a deadline on the “Substantial Completion” milestone, and the project is 10-days behind schedule. For unrelated reasons, the project manager has identified a need to examine the predecessor and successor logic paths around one task – “A3 Install Line D” – which is selected in the figure.

 

Task Inspector

We can first examine the task’s dependencies using MSP’s Task Detail Form and the Task Inspector pane.  The form shows that the task has two finish-to-start predecessors and two finish-to-start successors, but it provides no other schedule information for these dependencies.  Task Inspector identifies the second predecessor — ID 22 – A3 Install Line C — as the driving predecessor.  That is the predecessor whose logic is controlling the start date for our task.

This driving predecessor is hyperlinked, so clicking on it automatically selects the corresponding task in the task table and updates the details.  In addition, a new “Go back to” task link is added to the Inspector pane.  Starting from a specific ending task, the user can use these tools to “click-trace” backwards and forwards along the driving predecessor logic path to the selected task.

Click-tracing of driven successor paths is not possible using the Inspector pane, and predecessors that are hidden by filtering or outlining can’t be traced.  Moreover, neither external links nor resource-driving links can be traced.  Finally, the driving predecessors can be incorrect when relationships other than finish-to-start are used.

BPC Logic Filter – Task Logic Inspector

The Logic Inspector in BPC Logic Filter presents consolidated views of a task’s driving and non-driving predecessors and successors, including other relevant information like dates, resources, or user-selected fields.  Driving relationships are highlighted and listed at the top, while links to inactive predecessors are de-emphasized and listed at the bottom.  For logical significance, the ERF column indicates how far (in days) the predecessor is from controlling/driving the successor’s early dates, and the LRF column indicates how far the successor is from controlling/driving the predecessor’s late dates.  The latter is most useful when click-tracing to explore successor logic paths.  (The yellow-on-red color scheme in the figure highlights relationships that are driving in both directions.)

With the Jump buttons in the Logic Inspector windows, users can click-trace logic paths forward and backward through the project schedule.  Unlike the built-in tools, the Logic Inspector windows allow inspection and click-tracing of both driving and non-driving logic along successor paths, external (i.e. master/sub-project) paths, resource-constrained logic paths, and hierarchical logic paths.

Task Paths and Task Path Filters

Recent versions of MS Project (2013+) include Task Path bar styles for graphically highlighting predecessor and successor paths on the Gantt chart.  In addition, it is possible to use a simple “QuickTrace” macro (like the one linked above or one of the freeware settings in BPC Logic Filter), to identify the same set of related tasks.  In the next figure, the Path Predecessors bar style has been applied to color the bars of all tasks that are path predecessors of the selected one.  The QuickTrace macro (included in BPC Logic Filter) has also been used to highlight the same related tasks.

Alternately, QuickTrace can display the related tasks using a filter rather than a highlighter, as shown below.  In either case, the actual driving logic for the selected task is not apparent.  (And no, Total Slack does not define driving logic for this task.)

The next figure displays both the Path Predecessors (orange) and the Path Driving Predecessors (red-orange) bar styles for the selected task, and QuickTrace has been used to highlight the driving predecessors in the table.  A corresponding filter could also be applied.

Although generally less useful for analysis, corresponding bar styles and filters can be applied for the path successors and path driven successors of the selected task.

BPC Logic Filter – Advanced Tracing

With advanced logic analysis features, the relative float of all the predecessor task paths can be defined and displayed.  The following chart shows the entire project, with the predecessor paths of the selected task highlighted according to their path relative float in days.  (Path relative float indicates how much a particular task or path may be delayed before affecting the selected task; i.e. “days away from driving”.)  Path relative float is indicated numerically at the right side of each related bar.  Unrelated bars are de-emphasized and colored green.  The bar chart shown is similar to that obtained using the Task Path bar styles, with the key exception that non-driving predecessor paths can be ranked.

BPC Logic Filter allows one variation – bounded network analysis – to only show connections to a particular target task.  The figure below highlights the connections between the selected task, A3 Install Line D, and the target task, A2 Civil.

The previous two figures displayed the related task paths in-line and within the context of the overall project view.  A more focused view of the driving and near-driving paths is provided by applying a filter to hide all unrelated tasks, as shown here.

It is often useful to group and sort tasks to clearly display the chain of driving logic and associated near-driving paths.  The first group in the figure below – “BPC Relative Float (d): 0” – indicates the driving logical path for the selected activity.  The next two groups depict branching logical paths (one task each) that are 10- and 20-days away from driving the selected task.

Finally, when attempting to accelerate a task by shortening its driving logical path, Drag quantifies the maximum acceleration that may be gained by shortening a particular task along that path.  Drag is limited by the existence of parallel paths, as clearly evidenced by Tasks 6 and 11 in the figure below.  For focused acceleration of complex projects, the Drag metric can assist in prioritizing actions.

The examples shown here represent successively more powerful analyses of driving logic for an arbitrary task in a project schedule.  If that task were the final task or a key completion milestone for the project, then the resulting special case of the driving path would be the “Critical Path” for the project.

See a related video entry:

Video – Logic Tracing Example in Microsoft Project

 

Leveling Changes from MSP 2010 to MSP 2013

[I’ve slightly edited this old article – mostly figures.  After “upgrading” to Microsoft Project 2016, I can confirm that MSP 2016’s resource leveler appears similar to MSP 2013’s leveler in the simple case examined; both are substantially different from MSP 2010’s.]

Over the last few years, some tests of resource leveling algorithms in various software packages have been reported informally over at Planning Planet.

When using “default” conditions (i.e. without implementing any user-defined priority schemes), the general picture was that leveled schedules from MSP 2010 were significantly longer than those from MSP 2007.  There have also been reports that MSP 2013 (and MSP 2016, which seems to use the same leveling rules) produces even longer leveled schedules than MSP 2010.  Other software (Spider Project in particular) tend to produce shorter schedules under resource constraints.  I haven’t paid too much attention to these reports because a) I don’t use resource leveling often, and b) when I do use leveling, I always define some specific leveling priorities, so the “default” results are not so relevant.

Last week, a French planner (“Alexandre”) on PP noted some significant changes in default leveling results when migrating a simple schedule from MSP 2010 to MSP 2013, wondering what might be the reason for the changes.  I’m still on 2010, so I haven’t noticed the change in 2013.  While it’s clear that Microsoft has modified the leveling rules from version to version, they don’t publish the details, so addressing the “reason for the change” is just speculation.  My own speculation is that minor changes to the leveling algorithm were made to limit apparent changes to the pre-leveling “Critical Path,” even at the expense of extending the schedule.  For Alexandre’s specific example:

  • The project has a total duration of 7.5 days, and the pre-leveling (i.e. CPM) critical path runs through task 5.  Task 5 has 0 days of total slack, while task 8 has 2 days of slack.
Leveling Fig0
Figure 0. Pre-Leveling (CPM) Schedule – 2010
  • In resolving the resource conflict between task 5 and task 8, MSP 2010’s default rule gives greater scheduling priority to task 5 because:
    • The CPM schedule has it starting earlier;
    • It has lower total slack in the CPM schedule;
    • It has a longer duration;
    • It comes first on the task list;
    • It is marked as “Critical” (?);
    • Some other factors…(?).

(I have no idea what the relative contributions of these factors are in the scoring.  Some might be zero.)

    • As a consequence of leveling, task 5 (the “critical” one) is scheduled first, while task 8 is delayed, and the project is extended 1 day for a total duration of 8.5 days. Total slack is re-computed after incorporating the leveling delays, and a new “Critical Path” is displayed.
      • Perversely, Task 5 – which was clearly “critical” before leveling and is also clearly a resource driver for the project completion – now has 1 day of Total Slack.

        Leveling Fig1
        Figure 1. Leveled Schedule – 2010
      • Task 7 is now shown as “Critical,” even though there is neither a logical nor a resource-driving reason justifying it.
      • So in short, the MSP 2010 leveling algorithm can substantially change the “Critical Path,” and the resulting slack values can be completely misleading. (I wrote about this here: Logic Analysis of Resource-Leveled Schedules (MS Project).)
      • The next figures (from BPC Logic Filter) illustrate the actual resource-constrained Longest Path (Fig. 2) and Multiple Float Paths (2a) through the 2010 schedule.
Leveling Fig2
Figure 2. Resource-Constrained Longest Path of Leveled Schedule – 2010
Resource-Constrained Multiple-Float Paths of Leveled Schedule - 2010
Figure 2a. Resource-Constrained Multiple-Float Paths of Leveled Schedule – 2010
  • In contrast, MSP 2013 task gives greater scheduling priority to task 8, delaying task 5. I suspect this is driven by some complex tuning of the leveling rules around Total Slack.
    • Alexandre provided the MSP 2013 leveled schedule here (which also includes a minor date shift).

      Leveled Schedule - 2013
      Figure 3. Leveled Schedule – 2013
    • I’ve repeated it below in my [MSP 2016 model using Standard leveling order.]  The resulting “Critical Path” appears essentially the same as the pre-leveling version, but with an added 3-day leveling delay before task 5. This may give project managers confidence that the leveling exercise has not “screwed up” their critical path.
      Figure 4. Leveled Schedule – 2016

      Unfortunately, the project finish has been extended by an additional 2 days compared to the MSP 2010 leveler.  It is now 10.5 days.

    • The project manager’s confidence in the critical path is still misplaced. Task 7 and task 8 are now shown as far from critical, with 5 days of total slack.  As shown in the next two figures from BPC Logic Filter, however, they are obviously on the resource-constrained longest path through the schedule.
Figure 5. Resource-Constrained Longest Path of Leveled Schedule – 2016
Figure 5a. Resource-Constrained Multiple-Float Paths of Leveled Schedule – 2016

Comparing the 0-Float Paths of the two schedules (Figures 2a and 5a), we see that unlike MSP 2010, MSP 2013 and MSP 2016’s leveling engine has preserved the logic drivers from the pre-leveling CPM schedule while implicitly inserting task 7 and task 8 into the driving path to project completion.   Although the project is extended as a result, the appearance of a stable critical path is preserved.  Fortunately, BPC Logic Filter depicts the resulting resource-constrained critical path clearly in both cases.

My speculation on the reason for the changed algorithm is based on the Project development team’s demonstrated preference for the appearance of stability (for mid-level users) over behavior that might be more technically correct from an advanced user’s point of view.  (See the handling of inactive tasks in MSP 2013 for another example.)

For an update on this topic, see Resource Leveling Changes from MSP 2010 to MSP 2016 – Revisited

 

The Resource Critical Path – Logic Analysis of Resource-Leveled Schedules (MS Project), Part 2

BPC Logic Filter for Microsoft Project provides an automated method for extracting and presenting the Resource Critical Path (a.k.a Resource-Constrained Critical Path or Critical Chain) from a leveled schedule.

In a previous entry (Logic Analysis of Resource-Leveled Schedules), I investigated the impact of resource leveling on the logical analysis of Microsoft Project schedules.  Conclusions were not encouraging, i.e.:

  • Project’s Total Slack calculation – and as a consequence, the “Critical” flag – fails to adequately account for resource constraints in the schedule. Neither the “Resource Critical Path” nor any other resource-leveled logical path can be deduced from the schedule by analyzing the logical relationships and slack (nor even the “Leveling Delay” artifacts).  Even worse, tasks that are clearly schedule critical when considering resource constraints can have unexpectedly high values for Total Slack and may therefore be neglected during crashing exercises or disruption analysis.
  • BPC Logic Filter – our preferred tool for logical analysis of Microsoft Project schedules – could not completely overcome the weaknesses of MSP when it came to resource leveling.
  • To be amenable for logical analysis, it seemed that schedules needed to be constructed with “soft” logic links to mimic the impacts of the resource leveling algorithm.

As the developer and primary user of BPC Logic Filter, I was not satisfied with these latter conclusions.  After all, I had developed the tool specifically to overcome MSP’s shortcomings in the context of multiple deadlines/constraints and variable calendars.  Why should it stop there?

While I have intended to address resource constraints in BPC Logic Filter since the beginning, I didn’t have much need for it until recently.  Now the latest code revision incorporates full analysis and comparison of resource assignments in parallel with the existing logic analysis algorithm.  I’m pleased with the results and feel confident that BPC Logic Filter can now depict the Resource Critical Path (or any resource-constrained driving path) in a Microsoft Project Schedule.

Figure 8 of the earlier article showed the resource-leveled schedule of a simple construction project, while Figure 9 showed BPC Logic Filter’s multiple-float-path analysis of the schedule.  The latter figure demonstrated how resource leveling introduced gaps into the logical arrangement of the schedule, but it did not track the resource constraints behind those gaps.  Figure 11 of the earlier article demonstrated the analysis after replacing the resource-leveling delays with soft (“preferential”) logic links to create exactly the same (early) schedule dates.  I’ve included these figures below.

Figure 8: Resource-Leveled Schedule
Figure 8: Resource-Leveled Schedule
Figure 9: Logic Analysis of Leveled Schedule
Figure 9: Logic Analysis of Leveled Schedule
Figure 11: Logic Analysis of Schedule with Preferential Logic Instead of Leveling
Figure 11: Logic Analysis of Schedule with Preferential Logic Instead of Leveling

Now I’ve added a figure (let’s call it Figure 12) showing the “new and improved” analysis from BPC Logic Filter.  The top band of the figure illustrates the Resource Constrained Critical Path for the project as originally scheduled and leveled (i.e. without preferential logic).  The dates remain unchanged.  The relative float values for all tasks are identical to those of the revised (preferential-logic) schedule, but the total slack, critical flag, and bar colors are as originally scheduled and leveled.  The next figure (13) shows a revised version of the bar chart with custom bar colors applied to clarify the logic- and resource-driven paths.  This is comparable to MSP’s “Task Path” graphical display (though of course that tool is limited to logical paths, does not differentiate among relative float paths, and has no filter.)

Resource Constrained Critical Path from BPC Logic Filter
Figure 12: New Logic Analysis of Leveled Schedule – i.e. Resource Constrained Critical Path

Figure 13 - BPC Logic Filter Bar Colors

Figure 13 – BPC Logic Filter Bar Colors

To summarize, BPC Logic Filter now includes full analysis of driving resource constraints for leveled schedules.

I’ve tested the upgraded functionality against some of the more sophisticated leveling scenarios, like split-assignments, split-tasks, and in-progress tasks with splits.  I’ve also stress-tested the algorithms against some public-domain resource-constrained schedule datasets – namely PSPLIB files j1201_7 and j12060_10, both leveled by MSP with default parameters. For the latter project, BPC Logic Filter required nearly two minutes to chug through the numerous parallel resource-driving paths.  (The project includes large pools of homogeneous resources distributed among many small tasks, which I hope is not typical.)

For now, the resource-analysis features do not work across multiple projects (i.e. linked master/sub-project structures.)

Finally, the banner/featured image at the top of this article is an updated version of the project schedule, with a couple arrows added to depict the inherent logic of the resource leveling delays.  It would be great for the program to insert these arrows automatically, but I’m afraid the necessary effort isn’t justified at this time.  Maybe I’ll revisit in the future.

[See a related video entry here: Video-Using BPC Logic Filter to Analyze Resource Leveled Critical Path]