Calculating Expected Time Durations Using PERT-Representing and Scheduling Project Plans
Calculating Expected Time Durations Using PERT
One of the most difficult and most error-prone activities when constructing a project schedule is the determination of the time duration for each task within a work breakdown structure. It is particularly problematic to make these estimates when a high degree of complexity and uncertainty characterize a task. PERT (program evaluation review technique) is a technique that uses optimistic, pessimistic, and realistic time estimates to calculate the expected time for a particular task. This technique helps you obtain a better time estimate when you are uncertain as to how much time a task will require to be completed. The optimistic (o) and pessimistic ( p) times reflect the minimum and maximum possible periods of time for an activity to be completed. The realistic time (r), or most likely time, reflects the project manager’s “best guess” of the amount of time the activity will require for completion. Once each of these estimates is made for an activity, an expected completion time (ET) can be calculated for that activity. Because the expected completion time should be closer to the realistic time (r), the realistic time is typically weighted 4 times more than the optimistic (o) and pessimistic ( p) times. Once you add these values together, it must be divided by 6 to determine the ET. This equation is shown in the following formula:
For example, suppose that your instructor asked you to calculate an expected time for the completion of an upcoming programming assignment. For this assignment, you estimate an optimistic time of 2 hours, a pessimistic time of 8 hours, and a most likely time of 6 hours. Using PERT, the expected time for completing this assignment is 5.67 hours. Commercial project management software such as Microsoft Project assists you in using PERT to make expected time calculations. Additionally, many commercial tools allow you to customize the weighing of optimistic, pessimistic, and realistic completion times.