PERT Three Point Estimate
The PERT formula turns an optimistic, most likely, and pessimistic guess into a defensible expected value, weighting the middle opinion four times as heavily.
The equation
E = (a + 4m + b) / 6, σ = (b – a) / 6
- a and b are the optimistic and pessimistic estimates
- m is the most likely single value
- E is the expected value, σ a standard deviation
Worked example
An activity estimated at 8 days best case, 12 likely, and 22 worst case has an expected duration of (8 + 48 + 22)/6 = 13.0 days with a standard deviation of 2.3. Note how the long pessimistic tail pulls the expectation above the most likely value.
When to use it
Quick, transparent, and far better than single point guesses, but it assumes the three estimates are honest. Chains of activities need the variances summed along the critical path, or better, a full Monte Carlo of the schedule.
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