What is the PERT estimate formula and when is it used?

The Program Evaluation and Review Technique, or PERT, uses three estimates to calculate a weighted average duration for an activity: optimistic, most likely, and pessimistic. The formula is optimistic plus four times the most likely plus pessimistic, all divided by six. For example, if optimistic is four days, most likely is six days, and pessimistic is fourteen days, the PERT estimate is four plus twenty-four plus fourteen divided by six, equaling seven days. PERT accounts for uncertainty better than single-point estimates.