Beware inactive tasks in MSP 2013+, especially if you use non-finish-to-start relationships.
The other day Dominic Moss posted a nice Pulse article over on LinkedIn called Inactive Tasks – more than meets the eye. I liked the article, though I pointed out some fundamental issues with Inactive Tasks that were introduced in MSP 2013. [Note: the link takes you to a new, improved version of Dominic’s article; our original discussion was lost.] Here’s a recap of the lost comments:
“Dominic, this is a nice write-up. I was an early adopter of inactive tasks in MSP 2010 for the reasons you mentioned. With MSP 2013, however, Microsoft introduced a substantial change to schedule calculation around inactive tasks – the program inserts a hidden FS link between inactive task predecessors and successors at run time. I imagine that change persists in 2016. It seems a large number of users complained about the logical consequences of inactivating a task – i.e. its successors lose their driving logical constraints and slide back to the project start date. While the change seems to make life easier for the complainers, users should recognize that inactivating a task does not reliably remove it from the scheduling calculations (except in MSP 2010). That defeats most of their benefits for my purposes…. I stumbled across that ‘feature’ by accident, when a client’s planners (newly upgraded to 2013) started getting different end dates than I was (on 2010) from exactly the same mpp file. We had a number of contingent tasks that were inactivated – some with FF links. MSP 2013 was replacing those FF links with FS links and driving out the project completion!”
So the conclusion is – Beware Inactive Tasks on MSP 2013+.
[Edit: July 2018 – having finally upgraded a dedicated machine to MSP Professional 2016, I can confirm that the program’s updated behavior is a bit more refined than I had experienced with MSP 2013. In the absence of complications caused by lags and multiple calendars, the run-time adjustments generally comply with expectations of a “dissolved” activity. That is, relationships are applied through a zero-duration inactive task – at least on the Forward Pass (the calculations leading to the Early Dates.) This can still extend the scheduled project completion if the inactivated task… 1. was on the driving path to project completion, and 2. possesses reverse logic flow, as in the case of a driven finish and a driving start. Unfortunately, all this sleight of hand seems to wreak havoc on the Backward Pass and the corresponding Late Dates; if the inactive task has a parallel logic path, the late dates (and corresponding Total Slack values) of its predecessors are typically incorrect. Lags and multiple calendars make things even more complex. We’ll have to address all these behaviors in the pending upgrade to BPC Logic Filter, our MSP add-in.]
As is the case for Total Float/Slack, introducing variable task calendars can significantly confuse the understanding of Critical Path Drag in Project Schedules.
Critical Path Drag is a useful metric in CPM scheduling that Stephen Devaux introduced in his book Total Project Control, but it is not widely computed or used. Drag 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. Viewed simplistically another way: if Total Float/Slack represents how much a non-critical task may slip before becoming Critical; Critical Path Drag represents how much a Critical task may be compressed* before becoming non-Critical. This can be very useful in focusing a project acceleration (e.g. Crashing) exercise.
It’s clear that theoretically many components in a CPM schedule can possess Drag: activity durations, early constraints, and predecessor lags to start with. Activity calendars can also possess Drag. I originally incorporated Drag analysis into BPC Logic Filter with the aim of correctly analyzing the Drag possessed by each component, but I recently decided to at least temporarily set aside that idea for the following reasons:
Drag seems so rarely used in practice that using even a single number for each activity is a stretch for the typical user; parsing it out to its many components seems unhelpful to most.
Allocating Drag purely by passive network analysis is hard.
Most importantly, other Drag-computing software (namely Spider and Asta) seem to compute only a single figure for each activity. Allocating Drag to different components could conflict with these single-value metrics under some circumstances, leading to confusion and possible loss of confidence in the tool.
Now BPC Logic Filter provides an option to compute a single driving path Drag number for each task included in a logical analysis – using a brute-force, remove-and-recalculate approach. This Drag reflects the maximum acceleration of the project’s completion milestone (or any other selected milestone or task in the project) to be gained by modifying the task’s duration alone. A negative value indicates that the project is shortened by lengthening a particular task – this is reverse logic flow.
As is the case for Float/Slack, introducing variable task calendars can significantly confuse the understanding of Drag. Consider a simple project comprising three 1-day tasks in sequence, beginning on a Friday and with the first two tasks assigned the “Standard” calendar, i.e. 5dx8h. The concluding task “Activity 3” (perhaps a management task) is on a “Monday-Only” calendar.** As shown in the figure below, Activity 3 is scheduled to complete on the second Monday.*** Clearly, removing the duration of Activity 1 (or 2) will accelerate the Completion Task – Activity 3 – by seven calendar days, from the second Monday to the first Monday.
So what is the Drag of Activity 1? As shown by the combined figures below, the answer depends on the calendar used to quantify the acceleration. Measured according to Activity 1’s own calendar, a Monday-to-Monday acceleration is 5 days – indicating 5 days of Drag. That same Monday-to-Monday acceleration is only 1 working day according to the calendar of the Completion Task – resulting in 1 day of Drag. Using the Project’s overall calendar could provide different results altogether. For Activity 3 of this example, the Project Calendar is neither the Local Task Calendar nor the calendar of the Completion Task (also Activity 3); then it becomes like the first two tasks with a Drag of 5 days.
Which is the correct calendar to use? My own preference is to use the calendar of the Completion Task to quantify potential acceleration of the Completion Task (i.e. the middle option above), and this is the default selection in BPC Logic Filter. A detailed logical analysis would assign 1 day of Duration Drag to each of the tasks in the example and a further 4 days of Calendar Drag to Activity 3. There is no logical basis for adding these 4 days of Calendar Drag to any other task, but using a mismatched calendar in a brute-force Drag analysis does exactly that, as shown in the top and bottom options above. Those options remain in BPC Logic Filter to allow consistent comparisons with Drag calculations from other tools.
*In the absence of constraints, variable calendars, and certain relationship lags; the compression of a particular Critical Path task can lead directly to compression of the project. Then the idea that Drag represents the potential productive compression of the task itself (i.e. the mirror image of Total Float/Slack) can appear true. Such a simplified view should not be generalized for complex projects, however.
**Credit and thanks for this example go to Vladimir Liberzon of Spider Project. Spider is one of the few scheduling tools to compute Drag.
***Project correctly computes Total Slack of 4 days for the first two tasks, meaning they could each slip 4 days – according to their own calendars – without delaying the project. MSP marks them non-Critical. They are nonetheless on the driving path to project completion, as shown by BPC Logic Filter, and they possess Drag.
Total Slack as presented in Microsoft Project is not a valid metric for logical analysis of Summary Tasks. Users are advised to ignore it.
Underlying the “automatic” project scheduling in Microsoft Project are some simple mathematical processes that are commonly referred to as the Critical Path Method (CPM). CPM schedule calculation starts with a “Forward-Pass” through the schedule network to identify the earliest POSSIBLE start and finish dates that can be scheduled for each task – subject to its calendars, predecessors, and early constraints. Next – starting from the project completion – comes the “Backward-Pass” to identify the latest ALLOWABLE start and finish dates for each task, subject to its calendars, successors, and late constraints. For each task the difference between the late and early dates is called “Slack”.
“Start Slack” = Late Start – Early Start
“Finish Slack” = Late Finish – Early Finish
“Total Slack” = the lesser of Start Slack or Finish Slack (with some exceptions).
In traditional CPM, the “Critical Path” – i.e. that collection of tasks that controls the completion date of the project – is identified using Total Slack according to one of the following definitions depending on the complexity of the project and other factors:
Total Slack = 0; or
Total Slack = the lowest observed value.
Microsoft Project identifies critical tasks based on Total Slack <=0 (or some other threshold). [As noted in another post, this often leads to an incorrect identification of Critical Path for a project.]
Summary tasks are not anticipated in traditional CPM algorithms, so calculation and interpretation of Slack/Float for summaries can vary significantly between scheduling tools. In some (or most) CPM tools, summaries exist primarily to roll-up cost and schedule data (including slack/float) from the underlying sub-tasks. In Project, however, each summary task exists first as a task in its own right, with corresponding logical and hierarchical relationships to other tasks. This leads to common misunderstandings of exactly what is meant by “Total Slack” of a summary task in Microsoft Project.
In general, the following observations hold true for MSP 2010 Professional:
Each summary task inherits its early and late dates from its sub-tasks according to the following:
Early Start = the earliest Early Start of all the sub-tasks;
Late Start = the earliest Late Start of all the sub-tasks*;
Early Finish = the latest Early Finish of all the sub-tasks;
Late Finish = the latest Late Finish of all the sub-tasks*.
[* Behavior shown is for normal scheduling, i.e. schedule from project start with ASAP constraints.]
The Start Slack, Finish Slack, and Total Slack of the summary task are computed according to the inherited early and late dates.
Since Total Slack of the summary task is derived from up to four different, logically unconnected sub-tasks, it has no significance with respect to the Critical Path or any other logic path in the project schedule. This is illustrated in the figure above: Summary task B inherits its Early Start and Late Finish from task B1 and inherits its Late Start and Early Finish from task B4. The resulting 4 days of Total Slack shown for the summary task reflects the difference between the Late Finish of task B1 and the Early Finish of task B4 – completely meaningless in the absence of any logical connections between the two tasks. The 4 days of Total Slack for the summary task is also unrelated to the Total Slack values of any of its underlying sub-tasks.
If the summary task possesses logical successors – or if its parent or any outline-ancestor summary tasks possess logical successors – then the late dates can be further accelerated through hierarchical inheritance. The result is total slack that is sometimes zero or negative.
If the summary task possesses a manually-scheduled sub-task whose start date precedes that of any other sub-task under the summary, then the manual sub-task’s scheduled start date will be written to both the early and late start of the summary task, resulting in zero start (and total) slack.
Some tools have allowed summaries to display the “most-critical” (or lowest) float of the associated activities – which at least means something. Similar behavior may be obtained in Microsoft Project by inserting a custom duration field – say “Rolled Up Total Slack” – using a simple formula (“=[Total Slack]”) with “Rollup” set to Minimum.
Many modern MSP schedules incorporate variable calendars, deadlines, late constraints, or resource leveling; and the basic CPM assumptions no longer apply. Under such conditions Total Slack for any task – summary or sub-task – becomes unreliable for identifying the Critical Path or any other logical path through the schedule. That’s when a utility like BPC Logic Filter is needed.
Here I outline a method – including the ImposeZFF macro – to provide “just-in-time” scheduling of flagged tasks in MSP. I also include a separate macro, RemoveZFF, to restore the tasks to as-soon-as-possible scheduling.
Another missing feature for anyone coming to MSP from Primavera’s planning tools is the “Zero Free Float (ZFF)” constraint. (Primavera now calls this the “As Late as Possible (ALAP)” constraint – NEVER to be confused with MSP’s identically-named version, which is pretty useless for forward scheduling.) The ZFF constraint delays an activity as much as possible without impacting any successor activities. It is useful for scheduling just-in-time works and deliveries while preserving the correct logic flow through the project schedule.
Consider the simple project shown here. The overall project completion is limited by a constraint on task B. (I never use such mandatory constraints in practice but use it here to avoid having to add other extraneous logic to the example.) Task A must be completed before B, but for maximum efficiency it is desired to perform this work just-in-time. Because MSP has no other obvious feature to arrive at the desired schedule outcome, the scheduler is tempted to make task A a Start-to-Finish successor of B. Such an approach is unsound and is to be avoided for a number of reasons. All links in a logic-driven project schedule must reflect real logical constraints, and real Start-to-Finish constraints are extremely rare.
So what is the scheduler to do? Most situations are easily solved by adding logical milestones to the project schedule. For example, a logic-driven “Site Ready for Delivery” milestone as a Finish-to-Finish predecessor of “Eqpt Delivery” would be quite normal. In the very few occasions when suitable logical workarounds cannot be devised, then it may be useful to impose early-start constraints to achieve the zero-free-float objectives. For good control over the process, I use a custom flag field to designate specific tasks requiring ZFF constraints. Then I run a macro that automatically computes and assigns the appropriate constraint to all the ZFF tasks in the project at once. This makes ZFF constraints easy to audit, review, and justify.
For our simple project example, I inserted the Flag20 local field into the Gantt Chart entry table and called it ZFF. Then I marked task A with the ZFF flag and ran the macro. The macro automatically imposed a new early-start constraint on the task. As a result, the 12 days of Total Slack (like the 12 days of Free Slack) that previously existed have been consumed. Task A is now correctly shown as Critical.
If your project has a logical chain of tasks that are all ZFF-flagged, then you’ll have to re-run the macro several times (or add a simple iteration loop to the code. I have one but didn’t include it here for simplicity.)
[May 2019 Edit: Though I don’t use this macro, one user has reminded me that when used to delay a string of connected tasks, selecting the tasks in reverse order may be more efficient than the forward order first included in this article. I.e. use the “For i…” block shown here in place of the “For each t” block originally included. I might get around to modifying that code eventually.]
Dim i As Long
For i = ActiveProject.Tasks.Count To 1 Step -1
Set t = ActiveProject.Tasks(i)
If Not t Is Nothing Then
'If t.GetField(Application.FieldNameToFieldConstant(FieldName))....
'...
'...
End If
End If
Next i
The code for imposing the constraints is shown below. Drop it into a VBA module in your Global.mpt file (or in a specific .mpp file if necessary), then look for and run macro “ZFFImpose”. I normally use another function to find the ZFF flag field in a project, but that function was too big to include here. So if you want to use a custom flag field other than Flag20, just change the “FieldName = “Flag20″” statement in the code.
Sub ZFFImpose()
' Macro Coded 31-08-16 by Thomas Boyle PE PSP PMP.
On Error GoTo 0
Dim i As Integer
Dim t As Task
Dim L1 As String
Dim L2 As String
Dim SW1 As Boolean
Dim SW2 As Boolean
Dim SW3 As Boolean
Dim FieldName As String
'FieldName = FindField_Flag("ZFF") 'Function not included here
Fieldname="Flag20"
If FieldName = "" Then
MsgBox Prompt:="No ZFF Flag Field Found", Title:="ZeroFreeFloat Constraints"
Exit Sub
End If
CalculateProject
For Each t In ActiveProject.Tasks
If Not t Is Nothing Then
If t.GetField(Application.FieldNameToFieldConstant(FieldName)) = "Yes" Then
If t.FreeSlack > 0 Then
t.ConstraintType = pjSNET
If t.Calendar = "None" Then
t.ConstraintDate = Application.DateAdd(t.Start, t.FreeSlack)
Else
t.ConstraintDate = Application.DateAdd(t.Start, t.FreeSlack, t.CalendarObject)
End If
CalculateProject
L1 = L1 & t.ID & " " & t.Name & " " & t.ConstraintDate & vbCrLf
SW1 = True
Else
L2 = L2 & t.ID & " " & t.Name & vbCrLf
SW2 = True
End If
End If
End If
Next t
GoTo Finish
tEHandler:
MsgBox ("Failed to impose Zero Free Float Constraints")
Exit Sub
Finish:
CalculateProject
If SW1 = True Then MsgBox Prompt:="Imposed Early Start Constraints to remove free float:" & vbCrLf & L1, Title:="ZeroFreeFloat Constraints"
If SW2 = True Then MsgBox Prompt:="No free float to remove on tasks: " & vbCrLf & L2, Title:="ZeroFreeFloat Constraints"
If SW1 = False And SW2 = False Then MsgBox Prompt:="No tasks marked for ZFF", Title:="ZeroFreeFloat Constraints"
End Sub
What is done with the “ZFFImpose” macro is easily undone with the “ZFFRemove” macro shown below. This macro returns all ZFF-flagged tasks to “As Early As Possible” scheduling.
Sub ZFFRemove()
' Macro Coded 31-08-16 10:30 by Thomas Boyle PE PSP PMP.
On Error GoTo 0
Dim t As Task
Dim L1 As String
Dim SW1 As Boolean
Dim FieldName As String
FieldName = "Flag20"
'FieldName = FindField_Flag("ZFF")
If FieldName = "" Then
MsgBox Prompt:="No ZFF Flag Field Found", Title:="ZeroFreeFloat Constraints"
Exit Sub
End If
For Each t In ActiveProject.Tasks
If Not t Is Nothing Then
If t.GetField(Application.FieldNameToFieldConstant(FieldName)) = "Yes" Then
If t.ConstraintType = pjSNET Then
t.ConstraintType = pjASAP
CalculateProject
L1 = L1 & t.ID & " " & t.Name & vbCrLf
SW1 = True
End If
End If
End If
Next t
GoTo Finish
tEHandler:
MsgBox ("Failed to remove Zero Free Float Constraints")
Exit Sub
Finish:
CalculateAll
If SW1 = True Then MsgBox Prompt:="Removed Early Start Constraints from designated ZFF tasks:" & vbCrLf & L1, Title:="ZeroFreeFloat Constraints"
If SW1 = False Then MsgBox Prompt:="No tasks marked for ZFF", Title:="ZeroFreeFloat Constraints"
End Sub
[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.
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.
Figure 2. Resource-Constrained Longest Path of Leveled Schedule – 2010Figure 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 – 2016Figure 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.)
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.
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 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 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.
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 ScheduleFigure 9: Logic Analysis of Leveled ScheduleFigure 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.)
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.
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: 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.]
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 ManpowerFigure 3: Over-Allocation of Structural Erection ManpowerFigure 4: Over-Allocation of Crane for Structural ErectionFigure 5: Over-Allocation of Area 3 Specialized Mechanical Installation ManpowerFigure 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).
Figure 7: Resource Leveling Options
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:
Figure 8: Resource-Leveled Schedule
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 9: Logic Analysis of Leveled Schedule
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 ScheduleFigure 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.
