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Standard Deviation Calculation

#1 User is offline   Veena Mehra 

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Posted 14 January 2014 - 06:48 AM

A project manager made 3-point estimates on a critical path and found the following results:
Need help is calculating the below problem, found in one of the mock test. Pls. help :)




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.
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#2 User is offline   killzeghost 

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Posted 24 March 2014 - 03:33 AM

View PostVeena Mehra, on 14 January 2014 - 06:48 AM, said:

A project manager made 3-point estimates on a critical path and found the following results:
Need help is calculating the below problem, found in one of the mock test. Pls. help :)




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,
StDev( A ) = (24-12)/6 = 2 ; Var( A ) = 4
StDev( B ) = (14-8)/6 = 1 ; Var( B ) = 1
StDev( C ) = (27-15)/6 = 2 ; Var( C ) = 4
StDev( D ) = (28-10)/6 = 3 ; Var( D ) = 9
StDev( 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
StDev( Path ) = SQRT(27) = 5.2

Correct answer is 2.

This post has been edited by killzeghost: 24 March 2014 - 03:34 AM

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