PERT - Program Evaluation and Review Technique.If you’re going to take the PMP exam, you MUST remember this formula.  I’ve used it countless times in the real world and it works with surprising accuracy.

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

Optimistic time (O): the minimum possible time required to accomplish a task, assuming everything proceeds better than is normally expected. Pessimistic time (P): the maximum possible time required to accomplish a task, assuming everything goes wrong (excluding major catastrophes). Most likely time (M): the best estimate of the time required to accomplish a task, assuming everything proceeds as normal.

How does it work?

Obtain three time estimates (optimistic, pessimistic, and most likely) for every activity along the critical path.  Plug your numbers into the formula and then sum the totals.  Though people will challenge you, you WILL have a more accurate critical path estimate.

I will speak to “Standard Deviation of an Activity” and “Variance of an Activity” at a later time.   They both leverage the same values but in different formulas.