In Primavera P6, when we use “longest path” option to determine the critical path, the process to determine the longest path is as follows:
- The longest path activities are determined by reviewing the schedule backwards (starting from the last activity) through the driving relationships;
- If the relationship between two activities is driving, then both activities are flagged as part of the longest path;
- If the relationship is not driving, then the longest path will end at that “path”.
Let’s look at a simple example:
- The longest path activities are determined by reviewing the schedule from Activity F;
- Then it goes backwards, if the relationship between two activities is driving (Activity E and F, Activity C and E, Activity B and E), then all those activities are flagged as part of the longest path (the checkbox in the “Longest Path” column are ticked);
- If the relationship is not driving (Activity D and E, Activity D-3 and E), then the longest path will end at that “path” (the longest path does not go through Activity D and D-3).
In case we use mandatory constraint or there is out-of-sequence activities, the longest path might not go to the beginning of the project as illustrated in the picture below.
The mandatory start constraint was applied to Activity D-2. The driving relationships are established from the last activity (Activity F) to Activity D-2 only. All other “path” (through Activity B, C, D) are considered as non-driving relationships. So the longest path does not go to the beginning of the project.
The ability or function to base the longest path on the lowest amount of free float (the longest path will go through Activity B, C) rather than cutting it off if the free float is greater than zero is still not available. Hopefully, it will be supported in later version of P6.
The driving relationships are determined based on relationship free float and business rules defined by Primavera P6.
How does Primavera P6 determine a driving relationship between activities?
Khuong Do Blog 
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