Chief Prognosticator

PERT Averaging for Estimates

In estimating, PERT is an averaging technique used to estimate the duration or cost of an activity. It is commonly used in project management. The equation is simple:

Estimate = (Pessimistic + ( 4 x Realistic ) + Optimistic ) / 6
This way your different guesses on a task’s duration are taken into account. Let’s say a piece of work could take between 2 to 12 weeks to do, depending on efficiency, skill of resource, and other factors. But the most likely estimate, based on previous experience is 4 weeks. The PERT estimate would be:
(12 + (4 X 4) + 2) / 6 = 5 weeks.
Use 5 weeks and you have hedged your bet a little. This technique is welll accepted in the PM community and can be useful when estimates on tasks vary.