Lessons · Project management · three-point estimates and PERT
Three-point estimates: an honest number from three guesses
Ask for the optimistic, most likely and pessimistic durations; the PERT expected value is (O + 4M + P) / 6 and the standard deviation is (P − O) / 6.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
Asked how long testing takes, the tester says about two weeks, unless the build is bad, then four. That sentence is three numbers. Writing down only the two weeks loses the bad-build case, and it is the bad-build case that breaks the date.
How to think about it
Ask three questions: if everything goes right, how long; most likely; if it goes wrong, how long. Weight the middle one four times. Then compute the spread, because the spread is the honest part.
Worked example
O = 8 days, M = 10 days, P = 18 daysThree answers to three questions.
E = (8 + 4 × 10 + 18) / 6 = 66 / 6 = 11 daysThe most likely counts four times; the tails once each.
SD = (18 − 8) / 6 = 10 / 6 = 1.67 daysThe spread. About two thirds of the time the activity lands within 1.67 days of 11.
Range at 2 SD: 11 ± 3.3, so about 7.7 to 14.3 daysThe number to put in front of a sponsor who asks for a commitment.
Your turn
O = 4 days, M = 7 days, P = 16 days. Write the expected duration.
E = (4 + 4 × + 16) / 6
Solve one, graded on the server
The trap
Averaging the three. (O + M + P) / 3 is a triangular estimate and it over-weights the tails. PERT trusts the most likely value four times as much.