Again the 3 Standards deviations from the mean are used to calculate the pessimistic time. It calculates a weighted average instead of simple average. The weighted average of a m and b with m weighted 4 times as heavily as a and b.
PERT is a method of analyzing the tasks involved in completing a given project especially the time needed to complete each task and to identify the minimum time needed to complete the total project.
The sum of the optimistic pessimistic and most likely times. Like the critical path method a PERT chart will contain information like float between activities early startfinish dates and late startfinish dates. BETA probability distribution is what works behind PERT. The expected completion time E is calculated as below.