Formula for PERT - Pessimistic time and optimistic time

Question

I found PERT examples include a formula

(a + 4m + b)/6

where a= optimistic time , b= pessimistic time and m= actual time to complete the task

but how to calculate a and b? please help me as soon as possible

Answer 1

You don't calculate a and b, you estimate them. And m is usually the most likely time (estimate) not the actual time.

How do you estimate them?

That's a hard question... you need to consider the best possible scenario, and the worst. It takes practice and experience. You will likely get it wrong the first few times.

Answer 2

The most pessimistic (P) case when everything goes wrong The most optimistic (O) case where everything goes right The most likely (M) case given normal problems and opportunities

Pert Chart Formula: (O + 4M + P)/6