In a separate article titled Schedule Critical Path, I have pointed to four different settings (criteria) commonly used to calculate path criticality. These configurations and associated filters were supposed to be general knowledge to all planning and scheduling specialist and experts. However, this is an assumption that fails more than too often.
The reality of the matter is, more than a few have used the wrong schedule calculation settings and filters from day one. Unless persuaded to change, they will do so indefinitely.
Again, as in previous article, we refer to the primary path when talking about critical path unless specifically described as another.
The first one we will discuss is the longest path, followed by Total float equal or less than zero. Third on the line is the Path with the lowest Total Float (it can be positive, zero or negative), and last but not the least, one defined by the contract (the client’s definition of criticality or the “it depends” critical path).
Together, let us review how the Longest Path (LP) compares to the calculation using Total Float (TF). These are the two main approaches. Both represent the critical path and fundamentally cover how we would deal with the third and the fourth criteria.
Criteria 1: Critical Path is the Longest Path
A scheduling default and most commonly used critical path definition uses the longest path on the Schedule Option dialogue box. The Primavera scheduling tool can identify critical activities without specifying any total float value by merely saying “Yes” to the longest path filter and following additional setting prescribed by the tool.
It will identify the primary critical path from the last activity of the open schedule up to the earliest and back. The longest path is the longest path in the open schedule network from start to finish.
It is the path with the longest overall duration. Its duration is the project overall remaining duration.
Note the word “remaining.” If the project has not started yet, then the longest path is from the first activity to the last activity. If the activity has already started, the longest path is from the first remaining driving activity to the last activity.
LP is the path through the schedule network where total duration is greater than any other path.
Many planners and schedulers using Primavera do not know that they must follow a certain specific setting to identify the primary critical path when using the longest path as criteria
Choose “Free Float” for the Longest Path option. It is the right setting to define the critical float paths based on longest path. The most critical path will be identical to the critical path derived when choosing to define critical activities as Longest Path in the General tab (Figure 1).
Figure 1 – Schedule Options Using Longest Path as the Criteria
Figure 2 – Filter for Critical Path = Longest Path
The Primavera P6 Project Management Manual states that the activities on the longest path in the network are critical. The longest path follows the network of activities that have an early finish equal to the latest calculated early finish for the project.
Calculation traces all driving relationships for those activities back to the project start date in a multi-calendar project (Figure 3).
Figure 3 – Longest Path of Project XYZ-1
Criteria 2: Critical Path is Total Float < = Zero
The second method most frequently used is to filter critical activities by specifying a total float less than or equal to zero. Many agree that this method of identifying critical path is arguably more widespread.
AACEi RP 49R-06 (2010) rightly mentioned that all activities with negative float are critical. It is true even if other activities have lower negative float value.
The simple rationale: If any activity is contributing to the delay of project completion, then it must be critical. It does not have to be in the primary path nor directly contributing to the delay. If the project will miss an important a mandatory baseline milestone, then the tool generates a critical path from that milestone.
Primavera uses Total Float < = 0 as its default setting. Activities (including relationship lines) with a Total Float value of 0 (or less) are shown in red on the chart. If they did not turn to red, there is probably an existing project constraint. The planner/scheduler should check the “Must Finish By” field on the Project Window.
If not needed, just remove the “Must Finish By” project constraint and let the schedule flow freely. If for some reason it is needed to be there and the project is finishing earlier than the Must Finish By date, then there would be no critical path generated from the criteria TF < = Zero.
It is one of those occasions where the Longest Path delivers the critical path perfectly by showing an unbroken path along critical activities. This illustrates the true critical path, the longest path.
Oracle® Primavera Project Management Schedule Options
Primavera Users should observe the proper settings of the tool (Figure 4 and Figure 5) when scheduling and making analysis. Schedule options are Users Defined Setting. Planners, schedulers, schedule owner, or whoever has similar access in a common work environment can opt to have a slightly different configuration. If the project is not careful, this simple technicality becomes the source of confusion, unreliable output, and bad decision-making.
Figure 4 – Schedule Options for Critical Path TF < = Zero
Figure 5 – Filter for Critical Path TF < = Zero
The result points to a significant difference between the LP and TF criteria as illustrated in Figure 6.
After re-scheduling using TF < = 0 as the criteria and the primary critical path was identified, it have much of the same activities as in the Longest Path but not all of them. It was missing three activities highlighted in Figure 3.
Those three activities disappeared after TF is less than or equal to zero was used (Figure 6). Furthermore, an additional three activities were present that were not on the longest path. It is now very clear that using one over the other truly has an impact on the resulting critical path.
Figure 6 – Project XYZ-1 Critical Path TF < = 0
The degree and magnitude of the differences will govern the consequential effects. It will vary depending on what activities are affected. The planner/scheduler will have to detect what activities went missing and what new activities became part of the critical path (Figure 7).
Figure 7 – Critical Path TF < = 0
(showing what went missing compared to LP)
The project controls manager or the project manager has the prerogative over what CP criteria to use. Through the help of the lead planning/scheduling specialist, the project should be able to pick the best one.
- A project schedule riddled with constraints will most likely choose the TF criteria over the Longest Path.
- Hard-linked large interdependent schedules organized separately using Key Interface Milestones will do better with the TF criteria than using the Longest Path.
Our study shows that relying solely on the Primary Path is not the best. Mitigating the limitation of the primary critical path by considering multiple paths of critical and near critical activities is a good idea.
“Critical paths are actually more dynamic in nature than static. It constantly changes with each schedule update. Even activities with high criticality can switch path. Some might and some will end up as not critical.
What we can filter from our P6 schedule are static critical paths. They are practically just a snapshot of the path in time. That snapshot is equivalent to a singular iteration run, using a risk analysis tool or one update.
It is therefore not a clever move to rely on the static primary path with blind confidence. Always crosscheck with periodic schedule risk analysis.
The importance given by many project managers on static path is one of the reasons why they fail to have a good handle of their projects. The path is good only at the moment of inspection and do not offer even a short standing influence on the overall deliverables.
If this is the right way, I can merely pick any 200 activities because anyone of them can be critical the next time we look.
What a downer! It seems very ineffective and absurd to focus on the critical path produced by a singular run (Frago, R., 2015.Risk-based Management in the World of threats and Opportunities).
As with any other project controls activity, it is best to exercise caution before embarking on something that will become a precedent and eventually part of a routine.
Using the wrong setting repeatedly is counterproductive if not a complete loss of opportunity. Generating a result that does not represent the correct output is a waste of time and effort. It misleads the Project Manager and other Stakeholders to make the wrong decision.
In the big situation, a man’s blind belief usually ends in disaster.
As a reminder, if you are the project manager, do not stop a knowledgeable project person short when he points to the technical side of the tool. He might be unto something that will protect you and the project from going through lot of troubles.
Listen, listen, listen some more… and then softly digest the message.
1) Schedule Critical Path by Rufran C. Frago
2) Effects of Constraints on Critical Path by Rufran C. Frago
Rufran C. Frago – Author (Rev. 1, 18-Apr-16)
Rufran is the author of the book Risk-based Management in the World of Threats and Opportunities: A Project Controls Perspective.
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Related articles authored by Rufran Frago.
- Schedule Critical path
- Primer to Good Schedule Integration
- Project Schedule: P50, Anyone?
- Schedule Baseline Dilemma Part 1
- Schedule Baseline Dilemma Part 2
- 4D Scheduling Part 1: What is it about?
- 4D Scheduling Part 2
- 4D Scheduling Part 3
- Risks as a Function of Time
- Project Schedule: P50, Anyone?
- Mega-Projects Schedule Management and Integration
- Scaffolding Hours: What are they? Part 1
- Scaffolding Hours: What are they? Part 2
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- Your World, Our Risk Universe
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- and more…