Tom Boyle – Page 6

Float is not Schedule Contingency, Except when it is

I am writing this as a reminder to myself and to colleagues.

Total Float is NOT schedule contingency. That’s right: Total Float is NOT schedule contingency. I say this after the umpteenth conversation with a very smart and experienced contractor who regularly uses the terms interchangeably.

First, a couple references:

Merriam-Webster

Merriam-Webster defines “contingency” as a Noun thus:

1: a contingent event or condition: such as

a: an event (such as an emergency) that may but is not certain to occur

b: something liable to happen as an adjunct to or result of something else

2: the quality or state of being contingent

When budgeting projects, our usage of the term has evolved from an “allowance for contingencies”, through “contingency account”, to often a simple “contingency” amount – setting aside funds for uncertain costs or unknown risks in project scopes. Similarly, a “schedule contingency” exists as an explicit setting-aside of time to address uncertain durations or risks in project schedules.

AACE

AACE International (formerly the Association for the Advancement of Cost Engineering International) issued a Recommended Practice for Schedule Contingency Management (70R-12) that defines schedule contingency as, “an amount of time included in the project or program schedule to mitigate (dampen/buffer) the effects of risks or uncertainties identified or associated with specific elements of the project schedule.” Then the RP emphasizes, “Schedule contingency is not float (i.e. neither project float* nor total float).” Finally, the RP goes on to define Project Float (in a footnote) as, “the length of time between the contractor’s forecast early completion and the contract required completion.” A key theme of the recommended practice is to make schedule buffers explicit, visible, and allocated to specifically identified schedule risks.

* I might quibble with the “project float” exclusion, particularly in any case where risk mitigation is its primary motivation.

ASCE

The American Society of Civil Engineers recently published a new standard: ANSI/ASCE/CI 67-17 – Schedule Delay Analysis. Page 5 of this new standard provides a fairly long definition of “Float” that seems essentially correct with the pronounced exception of its first clause, “Float — The time contingency that exists on a path of activities.” Obviously, this is a direct contradiction to the AACE definition above. It also appears largely at odds with the rest of the definition in the ASCE standard.

DCMA

The Defense Contract Management Agency (DCMA) promulgated a 14-Point Assessment methodology for analysis of Integrated Master Schedules, which seems to have been widely adopted in some industries.

Point 13 of the DCMA’s 14-Point Analysis defines the Critical Path Length Index (CPLI) as:

Critical Path Length + Total Float

CPLI = ____________________________________

Critical Path Length

Here the “Critical Path Length” is essentially the time (in days) from the current data/status date until the forecast completion date of the project. The “Total Float” used in the ratio turns out to be the difference between the forecast completion date and the baseline/contract completion date – we otherwise know this as “Project Float.” (This is also the value of Total Float that Oracle’s Primavera P6 scheduling software displays on the Critical Path when a Project “must finish by” constraint corresponding to the contract completion date is assigned.)

The DCMA guidance provides a target value of 1.00, corresponding to a Project Float of 0. It goes on to suggest that a CPLI greater than 1.00 (i.e. Project Float is positive) is favorable while a CPLI less than 1.00 (i.e. Project Float is negative) is unfavorable.

NEC ECC

The NEC Engineering and Construction Contract (ECC), a collaborative contract form with increasing use in UK and Commonwealth countries, has formalized the use of two alternate terms for schedule contingencies:

Terminal Float

Terminal Float is the difference between the current Planned Completion date and the (Contract) Completion Date. It is not of a fixed size, is not allocated to any particular risk(s), and is reserved for the exclusive use of the Contractor. Notably, an acceleration of early activities that results in acceleration of the Planned Completion date also increases the Terminal Float. Terminal Float is equivalent to the Project Float of AACE International terminology.

Time Risk Allowance

Time Risk Allowance (TRA) “is made by the contractor when preparing its programme. These are owned by the contractor and provided to demonstrate that the contractor has made due allowance for risks which are the contractor’s under the contract. They must be realistic – unrealistic allowances could prevent the project manager from accepting the programme.” In other words, TRA is explicit time contingency buffer/allowance that is allocated to specific activities and risk events in the project schedule.

Clause 31 of the NEC Engineering and Construction Contract (ECC) requires the Contractor to show float and “time risk allowance” (TRA) on the programme (i.e. schedule). The NEC does not specify exactly how to “show” the TRA, though the demand that it be “realistic” has been used to justify a requirement that TRA be distributed through the project schedule based on explicit risk assessment (like the AACE RP). Typically, the TRA is included in padded activity durations, with the specific TRA portion explicitly noted in a custom field for each activity. In any case, the TRA participates directly (as a duration) in the CPM calculations for the deterministic project schedule, an output of which is Float. (Obviously, it must be excluded from any schedule risk simulations.)

Confusion

The mixed definitions have led to some confusion and contradictory recommendations from professionals.

The ASCE document on Schedule Delay Analysis implies that “time contingency” simply exists, manifested as Float, on a network path. This is far from the common use of “contingency” as an explicit and proactive allowance for risk.

In the NEC environment, there seems to be a persistent tendency to conflate TRA with various types of Float, with some reputable legal sources describing both Terminal Float and “Activity” float as “leeway the contractor adds” in case of problems. In some cases, contractors are being advised to explicitly mis-identify activity Free Float as Time Risk Allowance in their project schedules, thereby reserving that portion of the activity’s Total Float for themselves.

My View:

Total Float and its components, Free Float and Interfering Float, are artifacts of the Critical Path Method (CPM) for project scheduling; they are computed and applied for each individual activity in the schedule. Float essentially exists at the activity level, and its aggregation to higher levels of organization (e.g. WBS, Project, Program) is not well established.

In the simplest CPM case (i.e. absent any late constraints, calendar offsets, or resource leveling), Total Float describes the amount of time that a particular activity may be delayed (according to a specific workday calendar) without delaying the overall project. The calculation is made with no concern for risk mitigation, and its result is strictly due to the logic arrangement of activities. Those activities with Total Float=0 are those for which ANY delay will cascade directly to delay of the project – this is the Critical Path. Traditionally, activities with substantial Total Float are allowed to slip as needed to divert resources to the Critical Path. This does not mitigate the overall project schedule risk, however, since the Critical Path itself is not buffered. A distinct schedule contingency/buffer to address schedule risk along the Critical Path is necessary at a minimum. Risk buffering of non-critical activities is also needed to guard against the consumption of float by outside parties. Although contractors will often provide the necessary buffers by padding activity durations, the padding is rarely identified as such (except in NEC contracts). Either explicit schedule contingency activities or explicit identification of risk buffers that are included in activity durations are preferred. Regardless of the method chosen, clearly this is not Float.

Modern CPM schedules often include deadlines or late constraints and multiple activity calendars, and they sometimes include resource leveling. All of these features can substantially affect the computation and interpretation of Total Float, such that it becomes an unreliable indicator of the Critical Path or of any logical path through the schedule. Thus complicated, it is still not useful for risk mitigation.

Finally, because of the float-sharing language that seems common in modern American construction contracts, it is useful to distinguish legitimate schedule buffers (e.g. contingency or TRA) from the vaguely-defined “project float.” That is, where “project float” exists primarily for risk mitigation, one should consider explicitly identifying and scheduling it as a “schedule contingency” (or “reserve” or “buffer”) activity extending from the early project finish to the contractual project finish.

Here is a link to an interesting article from Trauner Consulting on the subject of Shared Float provisions.

http://www.traunerconsulting.com/wp-content/uploads/Manginelli-Shared-Float-Article-FINAL-docx.pdf

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.
    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).
  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

Eddies and Checkdams: Odd Structures in CPM Logic Flow

In logic-driven project schedules, the scheduled start and finish of each activity is determined by a “driving path” of predecessor activities and relationships. Driving logic is said to “flow” along this path from the earliest predecessor to the activity’s completion. In simply-modeled projects, this driving logic flow is one-directional and continuous, such that any delay (or acceleration) of a predecessor task is directly translated to a corresponding delay (or acceleration) of its ultimately driven successors. Thus, delaying a task on the “Critical Path” (the driving path to project completion) ultimately delays the project. More complex schedule models – i.e. those using other than finish-to-start relationship links – allow the driving logic flow to be checked or even reversed, so delay or acceleration of a given task may not have the anticipated result on other tasks or on the project as a whole. Such effects can be transient, appearing and disappearing in the course of a single progress update.

Some Cases

Last year I read a series of articles by Miklos Hajdu (Research Fellow at Budapest University of Technology and Economics) on the sometimes-unexpected consequences of certain relationships in logic-driven project schedules. I encountered them again a few months ago during an extended Linked-In discussion that Hajdu started relating to Drag calculation. (BPC Logic Filter is one of the few scheduling tools that actually sets out to compute drag, and we identified some areas needing standardization of definitions.)

Hajdu’s Articles laid out five basic cases where incrementally delaying or accelerating a particular Critical Path activity might not lead to the expected delay or acceleration of the overall project:

Normal Critical Activities (Expected Behavior – i.e. Lengthening task extends project; Shortening task shortens project.)
Neutral Critical Activities (Neither lengthening nor shortening the task has any impact on project completion.)
Bi-Critical Activities (Either lengthening or shortening the task always extends the project.)
Reverse Critical Activities (Lengthening the task shortens the project; Shortening the task extends the project.)
Increasing Neutral Decreasing Reverse Critical Activities (Lengthening the task has no impact on project completion; Shortening the task extends the project.)
Increasing Normal Decreasing Neutral Critical Activities ( Lengthening the task extends the project; Shortening the task has no impact.)

A more recent blog by consultant Pat Weaver, Critical confusion – when activities on the critical path don’t compute…… reviewed these cases and illustrated the consequences using time-scaled-logic diagrams rather than the simple fragnet blocks first used by Hajdu. With graphical images especially, Weaver has done a great job of clarifying the underlying logic flow and emphasizing the consequences of careless planning. His article is a good read. Unfortunately, his suggestion that competent planners should avoid creating any such constructs in their project schedules seems impractical for planners using P6 or Microsoft Project to schedule complex projects.

I had encountered all of these issues previously in various project schedules and and had focused on them quite a bit while developing the drag-associated parts of BPC Logic Filter – that’s where the “Negative, Positive, and Absolute” terminology below came from.

Driving Logic Flow of the Cases

At the start of a project (i.e. ignoring progress updates and Data Date), every activity is scheduled to be completed according to a path of driving logic (comprising predecessor activities AND relationships) extending from the project start milestone or some valid external constraint forward to the activity’s Finish. (For the particular case of the Project Completion activity, its driving path is synonymous with the “Critical Path” of the project.)

Identifying the “Driving Logic Flow” through any arbitrary activity along that path starts with the Relationship Free Float values of its predecessor and successor links. A Relationship Free Float value of 0 indicates a driving relationship, while a value greater than zero indicates a non-driving relationship.
The activity’s duration “participates” in the driving logic flow if (and only if) there are “Driven” and “Driving” relationships at opposite ends of the activity. Various combinations of relationship float have the implications in the table below.

Relationship Float and Driving Logic Flow

Keeping in mind that any changes to activity duration can immediately change the driving logic flow through the schedule and the associated relationship floats – such that an activity described by the first line of the table above may jump to the third line simply by adding a day to its duration – we can interpret the table as follows:

An activity with a Driven Start and a Driving Finish has a Positive duration participation, as the logic flows forward through the duration from Start to Finish. This is “Normal” in Hajdu’s articles; lengthening the task extends the project, while shortening the task shortens the project. This case seems to represent the vast majority of activities in typical schedules. BPC Logic Filter computes a drag value corresponding to the activity duration, any applicable constraints, and parallel (i.e. near-critical) paths.
Positive Duration Participation in Logic Flow (“Normal Critical”)
An activity with a Driven Finish and a Driving Start, on the other hand, has a Negative duration participation, as the logic flows backward through the duration from Finish to Start. This corresponds to Hajdu’s “Reverse-Critical” case; lengthening the task and thereby allowing it to start sooner ends up shortening the project, while shortening the task and thereby forcing it to start later ends up extending the project. BPC Logic Filter computes a negative drag value for these cases, partly to indicate the apparently perverse logic at work. While such tasks seem to be rare in baseline schedules, I have seen them arise during updating on fairly high-level integrated masters (i.e. not in construction).
Negative/Backward Driving Logic Flow through Duration – Starting Early and working slower Helps; Starting Later and working fast hurts.
For an activity with only a Driven Finish and Driving Finish (or Driven Start and Driving Start), then the duration is bypassed and has no participation in the driving logic flow. This corresponds to Hajdu’s “Neutral Critical” case; neither lengthening nor shortening the task has any impact on project completion, and BPC Logic Filter computes a duration drag of zero for task B. It should be noted that although “dangling starts” and “dangling finishes” are evident in the examples depicted below, they are unrelated to the zero-participation observed. Adding non-driving start predecessors and/or finish successors to Activity B would not change the conclusions.
Logic Flow Duration Bypass – Through Finish

Logic Flow Duration Bypass – Through Start
An activity with both driving and driven relationships at both ends (i.e. minimum Relationship Float = 0 in all four columns of the table) represents four parallel driving logic paths: 1) through the Start only; 2) through the Finish only; 3) forward through the Start, the Duration, and the Finish; and 4) backward through the Finish, the Duration, and the Start. In this case, the Duration Participation is “Absolute,” since any change to the activity duration (either positive or negative) results in positive (lengthening) of the overall path. There is no chance to accelerate the project here, so BPC Logic Filter computes a duration drag of 0. This case corresponds to Hajdu’s “Bi-Critical” case. I’ve added an additional predecessor and successor to the illustration fragnets below – mainly to indicate that its occurrence is not limited to ladder-logic structures. The combined “Positive” and “Negative” behaviors are obvious.
Absolute Participation = Positive/Normal on Lengthening, Negative/Reverse on Shortening
Finally, the four variations of “Limited” Duration Participation arise from the cases where three of the four “Relationship Float” columns are zero. They essentially represent various combinations of “Positive,” “Negative,” and “None” cases above, and they correspond to Hajdu’s “Increasing Normal Decreasing Neutral” and “Increasing Neutral Decreasing Reverse” cases. With any of these cases, it is only possible to Lengthen, never to Shorten, the overall length of the project by modifying the duration of Task B, so BPC Logic Filter computes a drag of zero.
Positive Limited Low Participation – Acceleration won’t help, but slippage will hurt.

Positive Limited Low Participation – Acceleration won’t help, but slippage will hurt.

Negative Limited High Participation – Starting early and working longer won’t help, but delaying start will hurt.

Negative Limited High Participation – Starting on-time and working slower won’t hurt, but starting later and working faster will hurt.

Logic Flow Conclusions

Ultimately, these types of odd logic structures seem to arise from two contributory causes:

The legitimate need for project planners to include in the project schedule no more detail than is necessary to plan and control the work (at the level reflected in the schedule). In Primavera P6 and Microsoft Project scheduling software – based on the precedence diagramming version of the critical path method (i.e. PDM/CPM) – this need is partly satisfied by consolidating many single activities representing simple tasks (with only Finish-to-Start relationships) into longer activities representing more complex work, connected with relationships other than Finish-to-Start. Common examples are Finish-to-Finish and Start-to-Start, often with time or volume lags. As a consequence, driving (controlling) predecessor logic can either “push” an activity (through its “Driven Start”) OR “pull” the activity (by its “Driven Finish”) – or both. Similarly, the activity may drive its logical successors through its “Driving Start” or its “Driving Finish.”
The continuous-activity assumption in the prevailing PDM software packages like P6 and MSP. That is, while the activity may be pushed and/or pulled by predecessor logic, the activity’s duration remains as a rigid connection from Start to Finish, neither stretching nor compressing (nor splitting into parts) in response to logical pressures alone. Consequently, the activity’s duration will be scheduled at the earliest continuous interval that satisfies the most stringent of its start/finish predecessor relationships. All other predecessor relationships will possess “relationship free float.”

Within P6 and MSP schedule models, using Ladder Logic to approximate progressive feeding of work volumes between largely parallel activities is a technique that effectively models the actual work interfaces. Yet it seems virtually guaranteed for such paired-activity ladder structures to encounter at least “neutral-critical” and sometimes “bi-critical” driving logic flow during updating. In my opinion this should be acceptable as long as the paired activities are effectively managed together.

Negative (“Reverse-Critical”) driving logic flow, however, reflects a case where the work being depicted is too complex to be represented by a single activity, and further breakdown is needed. Since it also provides an opportunity for the scheduler to sequester or otherwise manipulate float, the underlying logic structure may be indirectly prohibited by scheduling specifications. BPC Logic Filter presently flags reverse flows of driving logic (using negative drag) during the drag analysis.

The multiple cases where the driving logic flow effectively bypasses an activity’s duration (“Neutral-Critical”) appear to be a natural outcome of the scheduler’s intent. In addition, they seem consistent with the actual work interfaces in some construction projects, particularly where there are substantial variations in the production rates of parallel activities. While BPC Logic Filter doesn’t currently identify such cases, it seems reasonable to modify the Gantt-bar coloring routines in a future release.

Ladder Logic (Parallel SS/FF Relationships) in Microsoft Project

Microsoft Project does not allow users to create multiple logical relationships between the same two tasks. Project DOES ALLOW importing of such multiple relationships in XML files, however. If desired, users can directly edit XML versions of project schedules to introduce multiple logical relationships, and these relationships appear to be persistent and active in all subsequent schedule calculations.

Microsoft Project and Oracle’s Primavera P6 are the two dominant tools for complex project scheduling in North America, and I’ve been a regular user of both tools and their predecessors since the early 1990s. Aside from major issues related to data architecture, one of the key remaining differentiators from a user’s point of view is Project’s limit of one logical relationship between any two pairs of activities.

This limit can be a deal-breaker for longtime users of other tools who are dependent on scheduling workflows using “ladder logic”, or concurrent SS/FF relationships with lag. This kind of logic is used to represent sequentially-related activities whose overall characteristics allow them to proceed mostly in parallel. In construction, a simple example might include digging 1,000 meters of trench, laying 1,000 meters of pipe in the trench, and covering the trench. The most timely and profitable approach to the work is to execute the three tasks in parallel while providing adequate work space between the three crews whose production rates are well matched. Using ladder logic, it is possible to model the work using three relatively long duration activities with concurrent SS and FF relationships. Appropriate time lags are assigned to the relationships to represent the necessary space (or work volume) offsets between the activities. A similar approach can be used to schedule shorter-duration activities performed by successive crews at the same location, where availability of ready work front for the follow-on crew is the the primary constraint. [See also: Overlapping Tasks in Project Schedules.]

In my own experience, I’ve found ladder logic useful for effectively modeling the field approach while avoiding unnecessary detail in the schedule. Implementing ladder logic in Microsoft Project (MSP) requires the use of dummy milestones, complicating the logic and adding needless detail. I would prefer to avoid this workaround. Recently I learned that the one-relationship limit may not be irrevocable, and combined SS/FF links may be used to implement pure ladder logic in MSP (with some outside help.)

Figure 1 illustrates a simple fragnet (in P6) comprising three activities that are linked together with ladder logic. (This is just a test schedule; the implied work relationships are meaningless.)

Figure 1: Typical Ladder Logic in P6 — Figure 1: Typical P6 Ladder Logic with Concurrent SS/FF Relationships and Lags

Figure 2 illustrates the same fragnet (this time in MSP Pro 2010). Dummy milestones have been added to carry the logic through the Start-to-Start side of the ladder, so five activities are needed to schedule the same work that requires only three in P6.

Figure 2: Typical MSP Ladder Logic Using (Dummy) Milestones

Figure 3 demonstrates a fully-functioning MSP fragnet that incorporates dual SS/FF relationships without the aid of dummy milestones. The relationships appear to be fully functioning in the forward-pass and backward-pass calculations, and they are fully editable.* (Like all relationships in MSP, changing the predecessor or successor task requires deletion and re-initiation of the relationship, i.e. not an edit.) It appears, therefore, that multiple relationships between a single pair of tasks does not automatically break the schedule calculations in MSP Pro 2010. Unfortunately, the only way I’ve found to add these relationships is by directly inserting them into an xml version of the schedule file, then opening the file with MSP. (Similarly, P6 schedules that are transferred to MSP through the xml interface can manage to keep their redundant links.) Once opened, MSP appears indifferent to the presence of these externally-inserted dual relationships, but it continues to prohibit introduction of new ones, whether through the standard user interface or through direct manipulation of the underlying data in the open project.

*Interestingly, the only way to successfully edit the dual relationships is using the predecessors or successors list in one of the “task form” lower-pane views as shown here. Attempting to edit using the Predecessors tab of the Task Information dialog window or using the predecessors or successors column in a task table fails — with MSP deleting rather than editing the relationship.

Figure 3: Ladder Logic Loaded Through Edited XML File — Figure 3: Ladder Logic w/ Concurrent Links Loaded Through XML

Overall, I’m encouraged that Project’s longstanding limitation of one relationship per pair of tasks is not – apparently – an inevitable and unavoidable consequence of the program design. Nevertheless, inserting redundant relationships through xml editing hardly seems workable in general, and I doubt that I’ll be using this technique much going forward.

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 9: Logic Analysis of Leveled Schedule

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

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]

Find the Connections Between Two Arbitrary Tasks in a CPM Schedule

BPC Logic Filter includes a little feature called Bounded Network (Target Task) Analysis. The feature allows a user to identify all the logical path(s) connecting two arbitrary tasks (or connecting a group of tasks to a single task) in a Microsoft Project schedule. I wrote a few (very dry) pages on it beginning around page 6 of the documentation: Introduction to BPC Logic Filter

I developed this feature out of my own need to efficiently communicate task dependencies to project stakeholders. For example, a piece of major equipment is scheduled to arrive on-site (pre-assembled) on 1April, but it is not scheduled for handover until 1July. For the eventual equipment owner, it is useful to have a graphical depiction of all the tasks – and only those tasks – leading from arrival to handover during the intervening three months (e.g. setting in place, hookups, mechanical inspection, systems testing, commissioning, acceptance testing, endurance testing, training, etc.) While sometimes such tasks all share a common WBS code or custom field, it is rare that such codes correspond 100% with the logical chain(s) of required activities. A logic-based filter provides a clearer picture.

Within the last few days I participated in a LinkedIn discussion on how to develop a similar filter in Primavera (i.e. Oracle Primavera P6), and I’m embarrassed to admit that my initial suggestions (to use Multiple Float Path) were completely wrong. In fact, the quickest way to show the connections between two arbitrary activities is to create a logical loop between them, then try to reschedule the project. P6’s error handler will list all the connecting tasks.

P6 won’t generate the logical filter for you, but the user interface has a very handy feature of being able to add activities to a pre-existing view by simply clicking on the “Goto” buttons in the relationship windows. The list generated by the loop error will guide your selections.

Here’s an example, taken from a ~2000-activity schedule for an ongoing marine project. I have selected two activities whose relationship is not obvious, but which are indeed related.

Figure 1: Filter for 2 Arbitrary Connected Activities

(Using Multiple Float Path analysis of the electrical activity “Connect Paceco…,” I found the plumbing activity “New 4 PW….” activity on float path 168. I didn’t count the activities in float paths 2 through 167, but we need to exclude most of them without examination. MFP is clearly not the answer.)

As shown on Figure 1, I first add a circular successor relationship from the later activity, “Connect Paceco…” to its distant predecessor, “New 4 PW….) Then I try to re-schedule the project. If no error is generated, the two tasks are not related, or the predecessor/successor direction of the connections may be the opposite of what you expect. Figure 2 shows the expected error message, with the listing of the looping paths.

Figure 2: P6 Circular Logic Message (with my cheats)

Now use the list as a guide to attach the connected activities to the existing view.

The result after completing the loop:

Figure 4: Manually Constructed Filter of Path Connecting 2 Arbitrary Activities

You might be tempted to make this into a permanent filter by assigning some custom coding to the visible activities and then making the corresponding filter specification. That doesn’t seem to be worth the extra time to me unless I know for sure that I will be using this filter again. A pdf or screen shot may be all I need.

Many thanks to Khuong Do for raising this question in the Primavera group on LinkedIn. In addition, while the method of manually constructing logical filters by jumping through relationships has been around for many years, I thank Zoltan Palffy and Gail Adams for reminding me that it is still there in P6. Using the circular logic report is something I would never have thought of on my own. All credit to Mr. Gerry Smith in the Primavera LinkedIn group for that stroke of genius.

Just for comparison, I used a laborious process to export this project from P6 to Microsoft Project so that I could run the similar report from BPC Logic Filter. Here’s the result. Yellow and Orange highlighters identify the “Selected” and “Target” tasks respectively. (The P6-to-MSP export/import process is crude: Activity IDs were lost. Calendars were lost, so dates were corrupted. Logic came through with no problems, however.)

Figure 5: Target Task Output from BPC Logic Filter

Simple Macro for Computing (Duration) Drag in MS Project

Here is the BFDDrag macro, which implements a brute-force technique to compute Critical Path Drag for all tasks in the active project.

Drag and Drag cost are two useful metrics in CPM scheduling that Stephen Devaux introduced in his book Total Project Control, but they are not widely computed or used. Drag – coined as “Devaux’s Removed Activity Guage,” represents the individual contribution of any single schedule element (e.g. a task) to the overall duration of a project. By definition, only “Critical Path” tasks have it, and a task’s Drag indicates the total theoretical project acceleration that could be gained if the task’s remaining duration were reduced to zero. (Duration is not the only contributor to Drag – consider predecessor lags and constraints – but it is certainly the most prevalent.)

Drag can be useful in prioritizing activities for crashing (i.e. shortening) a project schedule. Consider the simple project illustrated in Figure 1. The “Critical Path” (ACFIJ – dark-red bars) is partially flanked by a near-critical path (BEH – hatched red bars) with a relative float (same as total slack here) of 2 days. The overall project duration can be shortened by accelerating critical-path tasks, and most experienced schedulers would be tempted to first consider modifying the two longest critical tasks (C and I). Accelerating either of them by more than two days yields no benefit, however, as the near-critical path then becomes Critical. Drag indicates this limit for each task – not so obvious in complex project schedules – in advance.

Figure 1: NLD59DRAG1 — Figure 1: Simple Drag Example

Below is a simple, brute force, macro for computing task drag (i.e. the drag of it’s duration) in Microsoft Project. The procedure cycles through each task in the project. If the task is a non-summary, incomplete task that is marked “Critical”, then the procedure sets its remaining duration to zero and measures the resulting acceleration (if any) of the project finish date. That value is stored in the Custom Field – “Duration5” as DRAG. You’ll have to change this in the macro if “Duration5” is already in use. I’ve also included a few comments in the code to guide you in modifying its scope. The procedure is logged in the “Immediate” window of the Visual Basic editor. After running this macro, Drag Cost could be separately computed for each task using a custom field formula that references the value in “Duration5”.

As noted, this is a “Brute Force” approach that relies on repetitive re-calculation of the CPM network by MSP’s scheduling engine. At two calculation cycles for each critical task, it may take a long time to chug through a very large project, but it will get there eventually. If your schedule has negative Total Slack, then MSP may interrupt the procedure to pop up a schedule warning window that you will need to click through (I have not added code to turn this warning off, but you can if you want). This warning window may not always be visible if MSP is not the active application – you’ll need to click on it occasionally.


Sub BFDDrag()

'       Coded by Thomas Boyle, PE, PMP, PSP on 8Jan'16
'       This is a simple brute-force procedure to compute Duration DRAG (Devaux's Removed Activity Guage") for
'       each Critical task in the active project.  The procedure cycles through each task in the project.  If the task
'       is a non-summary, incomplete task that is marked "Critical", then the procedure sets its duration to zero and
'       measures the resulting acceleration (if any) of the project finish date.  That value is stored in the Custom
'       Field - "Duration5" as DRAG.  The procedure is logged in the "Immediate" window of the Visual Basic editor.

    Dim Tsk As Task, ODur As Double, OFin As Date, TDrag As Double
    Dim StartNote As String
    Dim Counter As Long
        
    Application.CalculateProject
    StartNote = ActiveProject.Name & vbCrLf & Now() & " Starting Drag Brute Force" & vbCrLf
    Counter = 0
        Debug.Print StartNote
    'OFin is the finish date to be tested.  Default is the Project Finish.
    OFin = ActiveProject.ProjectFinish
    
    'Default computes DRAG for each task in ActiveProject.  To compute only for selected tasks,
    'then change "ActiveProject.Tasks" to "ActiveSelection.Tasks"
    For Each Tsk In ActiveProject.Tasks
        If Not Tsk Is Nothing Then
            If Tsk.Summary = False And Tsk.PercentComplete <> 100 Then
                Tsk.Duration5 = 0
                Counter = Counter + 1
                Debug.Print Counter & "Checking Task: " & Tsk.ID & " " & Tsk.Name
                
                If Tsk.Critical = True Then
                    If Tsk.Duration > 0 Then
                        ' Remove Task Duration and Check Finish Date
                        ODur = Tsk.Duration
                        Tsk.Duration = Tsk.ActualDuration
                        Application.CalculateProject
                        If Application.DateDifference(ActiveProject.ProjectFinish, OFin) > 0 Then  'Decreasing Normal Critical case
                            TDrag = Application.DateDifference(ActiveProject.ProjectFinish, OFin)
                        Else    'Decreasing Neutral or Reverse Critical case (Zero Drag)
                            'Test for Increasing Reverse Critical Case
                            Tsk.Duration = ODur + 1 'Adding one minute to Duration
                            Application.CalculateProject
                            If Application.DateDifference(ActiveProject.ProjectFinish, OFin) > 0 Then
                                'Increasing Reverse Critical case
                                TDrag = -1 * 5 'INDICATOR ONLY. Need further testing to find limit
                            Else
                                TDrag = 0
                            End If
                        End If
                        Debug.Print "Drag = " & TDrag / 60 / ActiveProject.HoursPerDay & "Days"
                        Tsk.Duration5 = TDrag
                        Tsk.Duration = ODur
                        Application.CalculateProject
                    End If 'Positive Duration

                Else
                    Debug.Print "Non-Critical"
                End If 'Critical Task Check DRAG
            End If 'Summary=No
        End If 'Not Nothing
    Next Tsk
    
    Debug.Print "Previous: " & StartNote
    Debug.Print Now(), "Finished Drag Brute Force"

End Sub

I started to include Lag Drag in the procedure, but modifying the lag value through code is a little more complicated. That will have to wait for another time.

I originally wrote a version of this brute-force macro to test the performance of the drag computations in BPC Logic Filter. Unfortunately, although BPC Logic Filter runs much, much faster for larger networks, it can fail to catch the Drag limitations caused by early start constraints and task calendar changes away from a given task. This simple macro is still the standard (for incomplete tasks).

[20Jan’16 – Edited code to catch “negative” drag for “reverse-critical” tasks. These are rare critical-path tasks that include a reverse flow of driving logic.

22Jan’16 – Further code edit to zero-out negative drag for all except the singular case where increasing task duration shorten’s the project. In that case, the code leaves a value of -0.01 days as an indicator only.]

[This macro computes Drag only with respect to the overall completion of the active project, not any specific completion milestone, and it uses the active project’s default calendar for quantifying Drag. If your true Critical Path is defined by some completion milestone that is not the last task in the project, then you will need a more powerful tool. This video demonstrates Drag calculation in BPC Logic Filter.]

Resource Leveling Breaks the “Critical Path” – Logic Analysis of Resource-Leveled Schedules (MS Project)

[31Mar’16: The latest release of BPC Logic Filter now includes resource leveling constraints in the logical path analysis. I’ve written another article to summarize and amend this one: The Resource Critical Path – Logic Analysis of Resource-Leveled Schedules (MS Project), Part 2 .]

Effective management of resources – i.e. planning, procuring, mobilizing, and deploying – is a core competency for successful companies in project-focused industries like construction. Most scheduling tools based on the Critical Path Method (CPM) – like Microsoft Project – can generate project schedules without resources, but they also include methods for assigning, analyzing, and “leveling” project resources. In this context, “leveling” means selectively delaying some work (compared to the CPM-based schedule) pending the completion of other, more urgent works that demand the same resources.

This simple description might imply that a certain logical/sequential relationship is imposed between two competing tasks (i.e. the “less urgent” work can only start after the “more urgent” work is finished with the resources) – sometimes called “soft logic”. Unfortunately, the leveling engine in Project 2010 does not appear to use, much less preserve, any such soft logic. Consequently, logical analysis of the leveled schedule – including interpretation of Total Slack to determine critical path or driving logical path – appears invalid.

Figure 1: Simple Construction Project with Resource Loading

Figure 1 is a simplified CPM model of a construction project involving multiple trades working in multiple areas. The model includes realistic resource loading, but the logical links have been limited to “hard logic” only (i.e. physical constraints). In other words, there is no preferential logic to guide the resource deployments. The default 5dx8h weekly calendar is universally applied, and a deadline of 25Feb’04 has been imposed. The unleveled CPM schedule includes a forecast completion that is nearly 3 months ahead of the deadline, but resources are severely over-allocated – the schedule appears unrealistic and needs to be leveled.

Specifically:

Three civil works tasks are running concurrently, but there is only sufficient manpower to run them sequentially. (Figure 2.)
Three structural tasks are also running concurrently, and these require both manpower (Figure 3) and a crane (Figure 4), which is the limiting resource. They must be done sequentially.
There is room to install the five separate processing lines concurrently in Area 3, but there is only enough skilled manpower to install them one at a time. (Figure 5).
An electrical change order has been approved in Area 2, but this requires the same specialized crew that is already working there. The Change-order work must be delayed (Figure 6).

Figure 2: Over-Allocation of Civil Works Manpower

Figure 3: Over-Allocation of Structural Erection Manpower

Figure 4: Over-Allocation of Crane for Structural Erection

Figure 5: Over-Allocation of Mechanical Installation Manpower — Figure 5: Over-Allocation of Area 3 Specialized Mechanical Installation Manpower

Figure 6: Over-Allocation of Specialized Electrical Manpower — Figure 6: Over-Allocation of Area 2 Specialized Electrical Manpower

It is a simple matter to remove the over-allocations by manually executing Project’s leveling engine using near-default conditions (Figure 7).

The leveling engine resolves the over-allocations by selectively delaying those tasks (and task resource assignments, if specified) which are judged to be lower-priority according to Project’s proprietary rules. Figure 8 illustrates the results of the leveling exercise:

The primary artifact of the leveling process is the “leveling delay” task property, which is in units of elapsed-duration (i.e. “edays”). The leveling delay is incorporated into the forward-pass schedule calculation, pushing the early start dates of the affected tasks. (Separate leveling delays can also be applied to resource assignments, which can extend task durations. This has not been done here and is generally not recommended when assigned resources are expected to work concurrently – e.g. Crane and structural erection crew.) Leveling delay is also incorporated into the backward pass, removing “phantom slack” from logically-connected tasks.
Through the task leveling delay, the civil, structural, mechanical, and electrical tasks have been re-scheduled sequentially.
Substantial Completion has been delayed until two weeks after the deadline, resulting in 10 days of negative slack on the milestone and its logical driving predecessors.
There is not an obvious (-10d) total-slack path from beginning to end of the project.

Figure 9 illustrates the use of BPC Logic Filter to determine the driving path logic of the Substantial Completion task after leveling. The driving path is comprised of four tasks and two milestones separated by gaps, and the intervals of the gaps are determined by the “leveling delay.” Unfortunately, this does not describe a “resource constrained critical path.” In fact, the obviously critical tasks without leveling delay – including the first (i.e. “A1”) Civil and Structural works and the A2 Electrical works – now have high values of total slack and are shown far from the critical path. Consequently, it is clear that logical path analysis – including any evaluation of Total Slack – is not consistent with the rule-based resource leveling algorithm used by Microsoft Project.

Figure 10 illustrates the un-leveled schedule, revised to include obvious preferential logic for avoiding resource conflicts. The resulting task sequences and schedule dates are identical to those of the leveled schedule seen earlier, but the associated total slack values and “critical” flags are substantially different. As shown in Figure 11, however, the logic paths are clear and consistent with the real resource constraints of the project. The “BPC Relative Float (d): 0” group appears to represent the true resource constrained critical path for the project.

Figure 10: Preferential (Soft) Logic in Unleveled Schedule

Figure 11: Logic Analysis of Unleveled Schedule with Preferential Logic

To recap, Microsoft Project’s proprietary resource leveling engine offers a convenient tool for resolving resource conflicts in project schedules, and this functionality seems heavily used and highly valued in some industries. It does not appear appropriate, however, for use in complex projects where formal logical sequencing of tasks – including identification of Critical Path or Critical Chain – is required. In particular, Project’s “Critical” flag will fail to accurately mark the critical path in a resource-leveled schedule. Consequently, a project specification that requires both a logic-driven schedule basis and heuristic resource leveling appears contradictory.

[Click here to proceed to the follow-up article: The Resource-Constrained Critical Path – Logic Analysis of Resource-Leveled Schedules (MS Project), Part 2 .]

Beyond the Critical Path – the Need for Logic Analysis of Project Schedules

This entry is intended to review the use of the Multiple Float-Path calculation option in Primavera Project Management (P6) and to offer a brief example of its use compared to BPC Logic Filter (for Microsoft Project).

Project schedules generated using the Critical Path Method (CPM) are commonly used to model – and to document – the project team’s plan for executing the scope of work. Such a plan normally involves identifying necessary activities at an appropriate level of detail and specifying the necessary sequential relationships between them. The output from the CPM analysis is a list of activities with associated durations, dates, and float values – this constitutes “the schedule”.

Unfortunately, the sequential relationships that ultimately drive the schedule (i.e. the logical “plan”) can be difficult to communicate or analyze for all but the simplest projects. This is because Total Float – the telltale indicator of logical-path connectivity in simple projects – becomes unreliable (or unintelligible) for such purposes in the presence of variable activity calendars or late constraints. As a result, complex schedule models lose both usefulness and credibility among project stakeholders unless schedule managers go beyond the simple communication of dates, durations, and float.

Multiple Float Paths

Oracle’s Primavera P6 software (P6) has always included an option to compute “Multiple Float Paths” when calculating the schedule, but many experienced planners seem unfamiliar with it. The option facilitates the identification of the “driving” and “near-driving” logical paths for a single selected activity. The selected activity can be a key project milestone that may or may not correspond to the end of the project, or it may be a simple intermediate activity of particular or urgent concern.

Figure 1 represents a simple project for construction and handover of a small utility installation – originally modeled in Microsoft Project and then converted to Primavera P6. (The model was developed primarily for illustrating the impact of calendars and constraints; the work techniques illustrated are neither typical nor ideal.)

There are contractually-derived late-finish constraints on the Construction Project Complete milestone (24Apr’15) and the final Project Acceptance milestone (29Apr’15). These constraints affect the late dates (and consequently Total Float) for these activities and (parts of) their chains of predecessors.
There is a late-finish constraint (25Feb’15) on the “Install Fence” activity (reason not known), with similar impacts on late dates and Total Float.
Activities are scheduled using a 4d x 8h work week (M-Th), except for the two initial milestones which utilize a 24-hour calendar, and the final two Customer Checkout activies which utilize a 5d x 8h workweek.
The “Notice to Proceed” milestone is constrained to start no earlier than 10:00 PM on 05Jan’15.
P6’s scheduling options are set to define critical path activities on the basis of “Longest Path” rather than Total Float, and the Gantt chart appears to properly display the Critical Path by this definition. Thus, the two initial milestones are marked as critical because they are driving the project’s completion, even though their calendars allow a higher value for total float.

Figure 1: (P6) Simple Construction/Handover Project

Although “Longest Path” appears to correctly identify the driving path to the project completion (the Project Acceptance milestone), the contractor is equally interested in identifying the driving path to the “Construction Project Complete” intermediate milestone.

In P6’s advanced schedule options, we select “calculate multiple float paths” ending with the “Construction Project Complete” milestone” (Figure 2). As a rule, we calculate the multiple paths using “free float” rather than “total float”, since the former option best mimics “longest path” behavior.* The default number of paths to calculate is ten.

* See “P6-multiple-float-path-analysis-total-float-and-free-float-options” for more on these options.

Figure 2: (P6) Schedule Option for Multiple Float Paths

Figure 3 illustrates the result of re-calculating the schedule then displaying a layout that arranges the activities by “Float Path” and sorting by “Float Path Order”. In this figure, “Float Path 1” is the driving logical path leading to the Construction Project Complete milestone. “Float Path 2” defines the first near-driving-path, “Float Path 3” defines the next near-driving path, etc. Each “float path” is essentially a discrete branch from the main, driving logical path. Obviously, Float Path 1 defines the activities that offer the most opportunity to accelerate the construction project (and maybe the most risk of extending it.) According to the figure, higher float paths tend to have higher values of total float, though the correlation is not universal.

Figure 3: (P6) Multiple Float Paths to Interim Milestone

Unfortunately, P6 does not rigidly distinguish between driving-paths and near-driving paths. That is, while float path 1 is always “the” driving path, float path 2 may designate another, parallel driving path or a path that is 2 days from the driving path. It is not obvious how far a certain numbered path may be from driving; that is, what is its “relative float” with respect to the end activity? You can try to estimate this manually by looking at start and finish dates of various related activities in the output. More rigorously, the relative float of each path can be computed by summing the “Relationship Free Float” of all the relationships between the given path and the end activity. [Jul’18 Edit: In certain cases P6’s path-selection criteria can relegate parallel driving-path activities – even Longest-Path activities – to high-numbered float paths that appear far from the driving path. I described this in a later article – Relationship Free Float and Float Paths in Multi-Calendar Projects (P6 MFP Free Float Option).]

Ongoing management of projects often requires what-if analysis of prospective disruptions, and P6’s MFP can be useful. For example, the subcontractor for the “Install Bus and Jumpers” activity may request early access to accommodate a staffing conflict. Running MFP ending with “Install Bus and Jumpers” will identify the driving path of predecessors for this work (Figure 4), assisting in the review and consideration of the request.

Blog151226Fig4 — Figure 4: (P6) Multiple Float Path to Install Bus and Jumpers

Figure 4 demonstrates the utter lack of correlation between Total Float and the driving logical path for any given activity in the schedule.

A Word about LOE Activities and ALAP Constraints (P6)

Depending on the scheduled dates, P6 automatically sets the relationships of LOE (level-of-effort) activities to “Driving”. As a consequence, P6’s Longest Path algorithm traces driving flags directly through LOE activities to their non-critical predecessors, and these end up – incorrectly – on the Longest Path. Fortunately, this error seems to be avoided in Multiple-Float Path analysis. MFP tracing correctly identifies only true driving logic and excludes LOE activities from the trace. (I’ve illustrated this in another entry HERE.)

Like LOEs, predecessor relationships from activities with ALAP (as-late-as-possible) constraints in P6 can be flagged as “Driving” based on their dates alone. Consequently, each ALAP-constrained predecessor creates a new parallel driving path to the selected end activity, and these paths are mapped in the MFP analysis. Since ALAP-constrained activities are rarely actually driving anything, it can be useful to filter them out from standard MFP layouts.

Multiple Float Path Analysis in Microsoft Project

(Microsoft Project provides neither Longest-Path nor Multiple-Float-Path analysis. BPC Logic Filter is an add-in that applies similar calculations to MSP schedules.) Figure 5, Figure 6, and Figure 7 illustrate the same steps as above, but this time executed on the Microsoft Project version of the schedule using BPC Logic Filter. In this type of analysis, the primary difference between P6 and BPC Logic Filter is that BPC Logic Filter explicitly computes and displays “Relative Float” (i.e. days away from driving) for each path. Thus two logical paths with the same relative float (i.e. parallel paths) are grouped together in BPC Logic Filter, while P6 assigns separate float paths. The MSP add-in also re-colors Gantt bars based on their path relative float with respect to the “selected” task.

Figure 5: (MSP) Simple Construction/Handover Project

Figure 6: (MSP) BPC Logic Filter – Multiple Float Paths to Interim Milestone

Figure 7: (MSP) BPC Logic Filter – Driving Path to Install Bus and Jumpers

Finally, BPC Logic Filter allows a more substantial evaluation of the upstream and downstream logic affected by the potential change to “Install Bus and Jumpers”. Figure 8 identifies the predecessor and successor paths for the selected task, all arranged according to their path relative float (shown at the end of each bar) with respect to the selected task. This illustrates that, while the selected work cannot be accelerated without violating (or modifying) its driving predecessor logic, it may be delayed by up to 12 days without affecting any successor work.

Figure 8: (MSP) BPC Logic Filter – Driving and Driven Paths for Intermediate Activity (Install Bus and Jumpers)

As a long-time Primavera user accustomed to MFP analysis options, I was continually disappointed when faced with the need for logical path analysis in Microsoft Project. I wrote BPC Logic Filter in part to cover this gap; now I find myself facing disappointment in the opposite direction.

Simple Macro for Listing Driving Predecessor(s) in MS Project

The Drivers macro reads and copies the ID of each task’s driving predecessors into a custom text field for the task.

One of the missing features for anyone coming to MSP from Primavera’s planning tools is the little mark designating the driving predecessor(s) in the predecessors list for each activity.

MS Project 2010+ kind of addresses this with a feature called “Task Inspector” that opens a pane and displays some scheduling factors affecting the selected task. One of the factors displayed is the “Predecessor Tasks:” table, which lists the predecessor(s) (if any) that limit the early start date of the selected task. The driving predecessor is inserted as a link, so it is possible to jump backwards through the driving logic of the schedule.

Since I use BPC Logic Filter, I haven’t had much use for Task Inspector. Still, it is occasionally useful to see a long list of predecessors and know immediately which ones are driving the selected task. Here is a little macro to copy the IDs of the driving predecessors into the “Text2” field of each task.

Sub Drivers()

    Dim t As Task
    Dim td As TaskDependency
    Dim PredDr As String
    Dim i As Integer
    
    
    For Each t In ActiveProject.Tasks
        If Not t Is Nothing Then
            PredDr = ""
            i = 0
        
            For Each td In t.StartDriver.PredecessorDrivers
                i = i + 1
                If i = 1 Then
                    PredDr = PredDr & td.From.ID
                Else
                    PredDr = PredDr & "," & td.From.ID
                End If
            Next td
            
            t.Text2 = PredDr
        End If
    Next t

End Sub

The user can then insert the “Text2” column in any task view. The image at the top of the post is a little schedule with the column inserted as shown and named “Driving Pred”.

The “Drivers” determination is done by MSP’s scheduling engine, and I have found it to be unreliable for driving predecessors that are not Finish-to-Start links. (Problems with Driving Logic in Task Inspector and Task Paths – Microsoft Project 2010-2016) Nevertheless, it’s better than nothing. Obviously, the macro needs to be re-run whenever the logic or task numbering changes….

If you are interested in a real logic trace of the driving path for the activity, then you’ll need a much more sophisticated macro (like this one) or a full-fledged Add-In like BPC Logic Filter – shown in this video. As a side feature, BPC Logic Filter includes a task Logic Inspector that displays driving and non-driving relationships for any task. Visit the BPC Logic Filter page to download a fully-functioning Trial version.