What is the expected time of completion of an activity, if the optimistic time (t₀) is 4 days, most likely time (tL) is 6 days and pessimistic time (tP) is 11 days ?
- A6·5 daysCorrect
- B9·5 days
- C3·25 days
- D4·25 days
Solution
Concept
PERT expected time
Formula
te = (t₀ + 4 tL + tP) / 6
Step-by-step solution
1. te = (4 + 4 × 6 + 11) / 6 = 39 / 6 = 6.5 days.
Explanation
PERT assumes a beta distribution, so the expected time gives four times the weight to the most likely time.
Why the other options are wrong
9.5, 3.25 and 4.25 do not equal (4 + 24 + 11)/6.
Shortcut
(a + 4m + b)/6.
Common mistake
Dividing by 3 or taking a simple average.
Correct answer: A) 6·5 days