Duration Uncertinity - Sigma

Submitted by Mac1 on Wed, 07/06/2011 - 20:29

Please guide on the following Q regarding Duration uncertnity - Correct answer is 2 (30 days)

Together with your team, you applied three-point estimation on a Critical path which consists of two activities. The following duration uncertainties are all calculated assuming a ±3sigma Confidence interval. The duration uncertainty—defined as pessimistic minus optimistic estimate—of the first activity is 18 days; the second estimate has an uncertainty of 24 days. Applying the PERT formula for paths, what is the duration uncertainty of the entire path? 94
o 21 days
o 30 days
o 42 days
o No statement is possible from the information given.

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rishiarien

Mon, 07/11/2011 - 08:55

First activity duration uncertainty : P-O = 18 days.


Therefore First activity sigma = (P-O)/6 = 18/6= 3


=> First activity variance = 9


Second activity duration uncertainty : P-O = 24 days


Therefore Second activity sigma = 24/6= 4


=> Second activity variance = 16


Entire path variance = 16+9 =25


Entire path sigma = 5


3 sigma = 5 x 3 = 15


Sigma is calculated from mean and uncertainty indicates the range


Uncertainty = 15 x 2 = 30 days