Once you have entered PERT values, you must ask Project to calculate the values before you can view each scenario type. Here’s why:

 

 

You can see that although we have entered optimistic, expected, and pessimistic durations for many sub-tasks, the total of those durations has not been entered in the summary tasks. When you click the Calculate PERT button () on the PERT toolbar, you will receive this warning:

 

 

This dialog explains exactly what the PERT calculation will do: recalculate your duration based on your expected, optimistic, and pessimistic estimates. It will also overwrite the first three Start/Finish fields. If Microsoft Project encounters any scheduling conflicts, you will be notified.