Describe how to calculate probabilities for project completion time using the Project Evaluation and Review Technique (PERT). PERT was developed as a means of 19.4 handling the uncertainties in activity completion times. The PERT procedure uses the variance in the critical path activities to understand the risk associated with completing the project on time. When activity times are uncertain, they are often treated as random variables with associated probability distributions. Usually three time estimates are obtained for each activity: optimistic, most probable, and pessimistic times. The expected project completion time is the sum of the expected times for the activities on the critical path. The variance in project duration is given by the sum of the variances of the critical path activities