Primavera Risk Analysis - Criticality Index and Project Risk When Critical Path is Not The Longest Path

In the previous post, in the end, this is what I wrote:

If you want to have the longest path as critical path and do not want to consider total float, then you can use this setting. However, the total float option is advisable. Why? It is because during risk analysis, the activities having negative total float will show a high "criticality index" as compared to the ones with zero total float. 

First, what is the "Criticality Index" of an activity?

As per Primavera Risk Analysis (PRA) software, 

During risk analysis tasks can join or leave the critical path. The criticality index expresses as a percentage, how often a particular task was on the critical path during the analysis. Tasks with a high criticality index are more likely to cause delay to the project as they are more likely to be on the critical path.  

If a task does not exist for some iterations (e.g. it is probabilistic) then it is marked as not being critical. For example a task that existed for 50% of the iterations and was critical 50% of the time it existed would have a criticality index of 25%.

In simple terms, criticality index is informing how often during the iterations, a particular activity/task is likely to be on critical path. 

Criticality index is used in Torando diagram. Remember - we are sampling and iterating many times with probabilistic duration, as noted in one of the earlier posts on Primavera Risk Sensitivity Analysis with Torando Diagram. 

Example:
Let me reuse the example of the previous post. There are 6 activities and 2 milestones with duration, free float, total float and the network diagram shown.

Critical Path - First Case

This is where I'll apply the concept of "Criticality Index". I imported this project (1st one without any constraint applied and without any negative total float) to PRA, applied a probabilistic distribution (triangular) and iterated 1000 times. With that, the criticality index comes as below.

Criticality Index (Tornado diagram) - First Case

Activity E and F have the highest criticality index, followed by C and D, as shown in the above Torando diagram. Or in others words, activities E and F have higher chances of delaying the project as compared to others, as they have  higher chances on being on the critical path.  Activity A and B are having criticiality index of just 2% or they are very less likely to delay the project.

Now I applied the constraint to activity B and changed the start date to before the finish date of its predecessor (as I had done in the previous post), the total float values changed as shown below. The critical activities also changed with activity A and B being new critical activities.

Critical Path - Second Case

When I imported to PRA and followed the same steps, i.e., applied probabilistic distribution and iterated - this is how the criticality index changed for all the activities.

Criticality Index (Tornado diagram) - Second Case

Above, you can see that the criticality index of the activity E and activity F remain same, i.e, at 86%. But for activity A and activity B, the criticality index is coming at 100%, which were earlier at just 2%. In other words, with the introduction of negative total float, the chances of the project being delayed by activity A and activity B are now the highest. 

Hence, it is important to note that critical path can be the longest path or critical path can also have activities with negative total float as well as zero total float. If you are not considering activities with total negative float, you are really ignoring the risks on your project. 

Tip:
1. Remember have proper setting in PRA for critical path post the import of XER file from Primavera P6.

2. It is not necessary to create the plans, set the dependencies, constraints etc in Primavera P6. All these can be directly created in Primavera Risk Analysis. You can fully do these operations - activities creation, linking, Gantt chart, applying constraints et al - directly in Primavera Risk Analysis. As most use Primavera P6 while creating a project plan, I have taken this approach. 

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