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### Standard Deviation

Posted: Fri Oct 06, 2017 5:21 pm
Q:A project manager made 3-point estimates on a critical path and found the following results:
Need help is calculating the below problem

Optimistic Most Likely Pessimistic PERT Weighted Avg
Act. A 12 15 24 16
Act B 8 9 14 9.7
Act. C 15 19 27 19.7
Act. D 10 14 28 15.7
Act. E 17 20 35 22

Estimate for the critical path: 83.1

Assuming ±3 sigma precision level for each estimate, what is the standard deviation of the allover path?

1. App. 4.2 days
2. App. 5.2 days
3. App. 6.2 days
4. You can not derive the path standard deviation from the information given.

Standard deviation of allover path is the square root of its variance.
Variance of allover path is the sum of variances of each individual activity.

Activity variance is equal to its squared standard deviation.
Standard deviation of a 3-point PERT estimate is (P-O)/6.

Thus,
Std( A ) = (24-12)/6 = 2 ; Var( A ) = std ^ 2 = 4
Std( B ) = (14-8)/6 = 1 ; Var( B ) = 1
Std C ) = (27-15)/6 = 2 ; Var( C ) = 4
Std ( D ) = (28-10)/6 = 3 ; Var( D ) = 9
Std ( E ) = (35-17)/6 = 3 ; Var( E ) = 9

Var( Path ) = Var( A ) + Var( B ) + Var( C ) + Var( D ) + Var( E ) = 4+1+4+9+9 = 27
Std ( Path ) = SQRT(27) = 5.2

Any Further Comments will be much appreciated!

### Re: Standard Deviation

Posted: Mon Oct 09, 2017 5:13 pm
Correct answer is 2 or option 2?

### Re: Standard Deviation

Posted: Tue Oct 10, 2017 8:22 pm
2 of course. Option 2 is 2.

Thanks!

Regards,