### Standard Deviation

Posted:

**Fri Oct 06, 2017 5:21 pm**Q:A

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

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!

**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 AvgNeed help is calculating the below problem

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!