Can you help with this PERT calculation

Submitted by akkarthik on Sun, 07/21/2013 - 20:39

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?
o 21 days
o 30 days
o 42 days
o No statement is possible from the information given.
 

how to canculate this?, which formula, pls advice. 

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ppalkonda1975

Sun, 07/21/2013 - 20:58

 I had put in the same post and one of the members posted the following answers

1 Calculate individual std dev

Formula: Std Devn = Duration Uncertainty / 6

2 calculate variance

For 1st activity, StdDev1 = 18/6 = 3   ==> Variance1 = 3^2 = 9

For 2nd activity, StdDev2 = 24/6 = 4  ==> Variance2 = 4^2 = 16

3.add variance

Variance of critical path = Var1 + Var2 = 9 + 16 = 25

4. calculate std for whole path back

Std Dev of critical path = sqrt(25) = 5

5. then calculate the uncertainty of critical path

 

==> Duration Uncertainty of critical path = 6*StdDev = 6*5 = 30 days

akkarthik

Sun, 07/21/2013 - 21:39

thanks a lot