The longest path through the schedule is a key concept. It tells how long the schedule takes. Further – activities along the longest path should be adjusted to support schedule compression and optimization efforts.
The term “critical path” is often used to refer to the “longest path”. But an activity that is critical does not necessarily mean it falls on the longest path. Well, in fact, there are two ways to define critical activities in Primavera P6 Enterprise Project Portfolio Management (EPPM).
- The first is by total float. Activities that have total float less than or equal to a defined amount of time, typically 0-days, are flagged as critical.
- The second is by longest path. Any activities that fall along the longest path are flagged as critical.
So an activity is critical either by its total float or by its position falling along the longest path. A longest path critical activity delay will postpone the entire project. However, a total float critical activity delay may affect either an activity constraint date or the project completion date. So a total float critical activity delay may or may not affect schedule completion: depending on the reason for its critical status.
Longest path critical activity delays are important to note as they will delay the entire project. And schedule optimization requires isolating the longest path through the network from among multiple critical and/or non-critical float paths.
This Primavera P6 EPPM release 16.1 article demonstrates how to isolate or unmask the true longest path from among multiple critical or non-critical float paths in support of schedule compression.
We have our demonstration project schedule in Figure 1.
This schedule has multiple float paths. It has an experimental path that is critical and an analytical path that is non-critical. It is obvious from the Gantt chart that the critical path shown in red is also the longest path through the network. But the longest path becomes less obvious when we insert a project constraint that falls before the natural network logic completion date, Figure 2.
Note the must finish by project constraint date in Figure 2. Now when we calculate the schedule we find that we have multiple float critical paths, Figure 3.
(Note the total float column and activities with negative total float.) The longest path is hidden among a sea of multiple float critical paths. Okay, our demonstration only has two multiple float critical paths, but you get the point. Our longest path is not readily apparent. We need to isolate the longest path from among the two float critical paths.
This is achieved by changing the definition of critical activities to longest path, Figure 4.
When we recalculate the schedule with this setting, we regain our longest path, Figure 5.
Great! This is what we want. We now know where to look to optimize our schedule. So when you have multiple float critical paths set the definition of critical activities to longest path in order to uncover the true longest critical path.
What about the situation where our longest path is one among many multiple float non-critical paths? To simulate this scenario we insert a (must finish by) project constraint that falls beyond the natural network logic completion date, Figure 6.
(Note we also reset the definition of critical activity to total float.) Our resulting schedule is in Figure 7.
Now our longest path is non-critical because all its activities have positive total float (observe the total float column), as per the total float critical activity definition and our project constraint. Now we need to isolate the longest path from among the two float non-critical paths.
Again, we achieve this by setting the definition of critical activities to longest path, Figure 8.
Our longest path appears one from among many non-critical float paths, Figure 9.
We are also relieved that we again have unmasked the true longest path through the network. And we can again optimize our longest path.
There is a simple lesson to learn from this longest path analysis. When you are in the optimizing-planning stages of your project, set the Primavera P6 EPPM schedule options critical activities definition to longest path.
Again, the longest path critical activity definition declares all critical activities as falling along the longest path. This uncovers the true longest path from a potential plurality of other zero float paths. And helps you focus schedule compression efforts on those activities that are directly driving the project completion date.