Software Estimations - PERT and adapted PERT

PERT comes from Program Evaluation and Review Technique and it's a common practice in the software estimations world, used in both bottom-up and top-down estimates.

The main principle is to give 3 estimates for each functionality/feature/module, etc. that has to be implemented:
1. the Best Case estimate : this is the most optimistic estimation; it would occur if and only if everything goes perfectly (no risks, no issues, no late dependencies, no lack of resources, etc)
2. the Most Likely Case : this is an estimated time based on another estimation technique ("Count, Compute and Judge", "Industry, Historical or Project Data Calibration", "Expert Estimation", etc)
3. the Worst Case estimate : this is the time needed if everything would go wrong (identified risks become issues, for example)

The original PERT formula for computing the Expected Case estimate is:

Expected Case = [Best Case + 4xMost Likely Case + Worst Case]/6

Usually, the estimators (like any other human being) are more on the optimism side and, more than this, they use to identify in the very beginning only few of the possible risks and dependencies. This why an adapted PERT formula has been put in place (and I recommend it's usage especially when the estimates come from engineers):

Expected Case = [Best Case + 3xMost Likely Case + 2xWorst Case]/6

References:
Steve McConnell - "Software Estimations - Demystifying the Black Art"