The optimistic, most likely and pessimistic time estimates of an activity are 5, 10 and 21 days respectively. What is the expected time and standard deviation?
- A12, 3
- B11, 4
- C11, 2.67Correct
- D10, 16
Solution
Concept
PERT mean and σ
Formula
TE = (a + 4m + b)/6; σ = (b − a)/6
Step-by-step solution
1. TE = (5 + 40 + 21)/6 = 11 days. 2. σ = (21 − 5)/6 = 2.67 days.
Explanation
The expected time and the standard deviation follow from the beta-distribution formulas.
Why the other options are wrong
Other options do not match both values.
Shortcut
(a + 4m + b)/6; (b − a)/6.
Correct answer: C) 11, 2.67