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Critical Path vs Longest Path in Primavera P6

Many of us get confused between Critical Path and the Longest Path when we look for a way to define a critical activity in Primavera. In fact, both terms are often interchangeable and there are several distinctions between Critical Path and the Longest Path of the Project and we will try to explain them here.

What is Critical path?

Critical Path is a longest continuous path of activities from the Progress Data Date to the Final Completion Activity, which determines overall completion date of the Project. The activities that typically comprise the Critical Path are those activities with the least amount of Total Float.

Please note that Critical Path is TRUE only in certain conditions mentioned below:

• No Project Deadline (when we set a deadline on our project i.e. Must Finish By Date in Primavera, it results in negative float if you are late and positive float if you are early)

• No Constraint (Constraints affect Total Float Value)

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What is the Longest Path?

The Longest Path is the path through which a Primavera P6 project network from start to finish where the Total Duration is higher than any other path.

Defining Critical Activities in Primavera

Please go to the Project view, select a Project, and click on Setting tab in the bottom layout:

This is the default behavior that that will be reflected in the schedule when running the schedule function.

However, this can be overridden at any time in the Schedule Options dialog (accessed by clicking F9 Key or select Schedule from Tools menu).

Difference between Total Float and Longest Path

Total Float value looks at the Total Float calculation of each activity in the network. If the Total Float value is ZERO, then it will be flagged as a Critical Activity. However, it does not necessarily mean it is on the longest path. It just tells us that activity is critical due to various factors.

We can out it this way – the Longest Path is the Critical Path, but not all Critical Paths are the Longest Paths.

The below Schedule shows how Total Float Method calculates the Critical Path where Activity A1050 has “Finish On or Before” Constraint that makes this activity and its predecessors critical. Therefore, it won’t impact Project Finish Date but it will overrun the Constraint Date.

However, if Longest Path method is used, the same set of activities will not appear as critical activities, and due to the constraint applied on Activity-A1050 along the path, the Project Finish Date will not be changed and they will not stay in the longest path.

Please note that previously critical activities are no more critical despite the fact that they have zero Total Float, but P6 ignores them because they are no more in the longest path anymore.

Conclusion: Which one should be used?

In my opinion, a Longest Path should be used to monitor for the timely completion of the Project.

Longest Path helps us to identify the activities that are driving the schedule finish date.

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