Oliverlehmann Question 94 Explanation Required

Submitted by anurap on Tue, 10/13/2015 - 18:01
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 possible from the info provided
 
Note: I got answer as A, but in answers it's B...can someone please expalin.
 
My calculation:
 
Task#1: (P-O) ==> 18 ==> Std Deviation==> (P-O)/6 ==> 3 ==> ±3sigma 3*3==> 9
Task#2: (P-O) ==> 24 ==> Std Deviation==> (P-O)/6 ==> 4 ==> ±3sigma 3*4==> 12
 Total: (P-O) ==> 42 ==> Std Deviation==> (P-O)/6 ==> 7 ==> ±3sigma 3*7==> 21
 
Forums: 
General Discussion
  • Log in to post comments
  • 124 reads

brian mahon

Wed, 10/14/2015 - 01:07

Oh Man this is about the nastiest question I have seen!

You need to know a few things before you answer it.

A.) The activity variance is the activity std dev squared.

B.) The Project variance is the sum of the activity variances. Which would make the Project std dev the square root of the variance.

OK so off we go.

We have Activity variances of 18 and 24 -> Square Each gives 324 and 576 respectively. Sum them to get the Project Variance of 900. Take the square root of the Project Variance to get 30 for the project standard deviation.

I know math so I can do it, but it's not really a fair question in my opinion.

B.