PMP question

Submitted by rwmv on Wed, 05/15/2013 - 03:21

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 statement is possible from the information given.

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crushPMP

Wed, 05/15/2013 - 03:31

P-O/6 = 3*STD

P-O for Activity-1 given as 18

18/6 = 3*STD

STD = 1

Var = 1

P-O for Activity-2 given as 24

24/6 = 3*STD

STD = 1.33

Var = 1.777

Var = Var1+Var2 = 1+1.7777

STD = SQRT(Var) =1.666

 

P - O / 6 = 3*1.666

P - O = 29.988

Ans = B?

 

BK

Wed, 05/15/2013 - 09:10

 A. 21 days?

Calculation done:

18/6+24/6=7

3 sigma=7*3=21

 

pmpnewster

Wed, 05/15/2013 - 09:34

B] 30 days

 

For Activity 1:   Sigma1 = 18/6 = 3  => Var1 = 3*3 = 9

For Activity 2:   Sigma2 = 24/6 = 4  => Var2 = 4*4 = 16

 

For total path, i.e. for Activity1 + Activity2,  Var1and2 = 9 + 16 = 25 =>  Sigma1and2 = sqrt(25) = 5

Thus duration uncertainty for total path = 5*6 = 30 days

 

Regards.