[Jason_sfh]

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?

[A] 21 days
[B] 30 days
[C] 42 days
[D] No statement is possible from the information given.

This post has been edited by Jason_sfh: 07 February 2012 - 07:07 PM

[raulcid]

The correct answer, in my view, is B: 30 days. This the way I would solve the problem:

1. Calculate sigma, the standard deviation, for each activity using the PERT formula

sigma= (pessimistic - optimistic) over 6.

We get a sigma of 3 for the first activity, and a sigma of 4 for the second activity.

2. Calculate the variance for each activity using the formula

variance = sigma^2

We obtain a variance of 9 for the first activity, and a variance of 16 for the second activity

3. Add the variances of the two activities to obtain the total variance in the critical path.

We get a total variance of 9+16 = 25

4. Take the square root of the total variance in order to obtain sigma for the project

sigma for the project = square root of 25 = 5

5. Calculate the duration uncertainty of the entire path assuming a plus minus 3 sigma interval, that is an interval of 6 sigma.

We get confidence interval for the entire project = 6 x 5 = 30.

[Deep Fried Brain]

Raul,

Your calculations look correct to me. Interesting question.

Best regards.