Deanna discusses how, specifically, to estimate task duration in her two-part series, "Project 2007: Estimating Task Durations using the PERT Formula." In this article, she describes options for calculating the task duration manually using the formula, and then how to do it in Project 2007. It is vital to understand how the PERT Formula works, and be able to do it by hand before allowing Project to calculate these duration estimates for you.
Here, I will discuss some of the methods used by Project 2007 in PERT analysis, and how these analyses can be helpful to a project manager. In addition to using the manual method for calculating duration estimates, Project 2007 offers two ways for PERT calculations. The first way involves using default information to calculate durations. The second manner of using the PERT Formula in Project is to use your own estimated probabilities.
The default probabilities included in Project 2007 for the likelyhood of the optimistic, most likely, and pessamistic estimates are, in order, 1 out of 6, 4 out of 6, and 1 out of 6. This is why, when you use the PERT Formula you divide the sum of the optimistic estimate, the most likely(4) and the pessimistic estimate by six.
To use the default probabilities in Project to calculate estimated duration, click on the "View" toolbar and select "PERT Analysis." After you do this, select the PERT Entry Sheet and enter in the estimates for pessimistic, optimistic, and most likely. Then, click Estimate PERT. Project 2007 will then perform the PERT Formula calculation. You can now also view Gantt charts corresponding with the optimistic, pessimistic, and most likely scenarios.
Sometimes you're pretty sure that the most likely scenario will win out - say you give it a 4 out of 5 probability and the pessimistic and optimistic probabilities are each 1 out of 10. Othertimes, a significant amount of risk plays into effect with your project schedule, making it so that the probability of the pessimistic view is higher. Project 2007 can be adapted to deal with this ambiguity. To do this, open the PERT Analysis toolbar as you would when using the default probabilities. This time, click on Set PERT Weights. Adjust the weights as needed, and then proceed as you did with the default probabilities.