I want to investigate and write about how to use PERT effectively in MS Project. But first I’ll do a review of the basics of PERT.
PERT was first developed within the United States Navy’s Polaris submarine program as a tool to help manage that very large, very complex, and unprecedented undertaking.
The term stands for Program Evaluation Review Technique. The focus of this management tool is on evaluating and forecasting the likely duration of a project.
Its distinguishing characteristic is recognising that project tasks are not deterministic: you cannot say with absolute confidence that task A will finish in 3 days, task B will finish in 5 days and so on.
The reality is that task A might finish in 2 days if everything goes well, might finish in 3-4 days in most circumstances, but can potentially take 8 days if some problems arise. So any project schedule crafted on the basis that A will take 3 days and B will take 5 days is simplistic, and is not based on reality.
To properly use PERT, you need to provide 3 duration estimates for each task in your network of activities:
- The optimistic duration (TOptimistic)
- The most likely duration (TMost Likely)
- The pessimistic duration (TPessimistic)
The optimistic duration is our estimate of how long a task will take if everything goes well. If we have previous experience of finishing a similar task where everything went according to plan, we can use that historical basis for our estimate.
The most likely duration is our estimate of how long the task normally takes, given the normal disruptions of normal execution.
The pessimistic duration is our estimate of how long the task will take to complete if things just are not working out. We can also use historical data if available. We’ll need to put an upper bound on this estimate because clearly there is no limit to the kinds of things that can go wrong, and we cannot include all of them in the estimate. If the task was painting the exterior of a house, we might consider such things as the impact of rain suspending the work, or of one worker calling in sick, or even running out of paint and the accompanying delays of buying more paint.
Once we have gathered the 3 durations, we use the following formula to come up with the most likely duration of the task:
TLikely = (TOptimistic + (4 * TMost Likely) + TPessimistic) / 6
Example:
| TOptimistic | 20 days |
| TMost Likely | 25 days |
| TPessimistic | 35 days |
Then
TLikely = (20 + (4 * 25) + 35) / 6 = 25.83 days
And we will use 25.83 days as our estimate for this task.
We perform the above procedure for each task in the project network, computing the TLikely duration for each.
