## Standard Deviation

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nazrulq
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Posts: 268
Joined: Sun Feb 26, 2017 6:06 am

### Standard Deviation

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

Correct answer is 2.

Any Further Comments will be much appreciated!
nirajsys@gmail.com
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Posts: 74
Joined: Tue Jul 11, 2017 9:28 am

### Re: Standard Deviation

Correct answer is 2 or option 2?
nazrulq
Contributor
Posts: 268
Joined: Sun Feb 26, 2017 6:06 am

### Re: Standard Deviation

2 of course. Option 2 is 2.

Thanks!

Regards,