PERT

Submitted by ppalkonda1975 on Thu, 07/18/2013 - 01:34

 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? 94

o 21 days
o 30 days
o 42 days
o No statement is possible from the information given.
 
whats the answer and explaination please
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ppalkonda1975

Thu, 07/18/2013 - 14:48

 anyone can help me with this one?

vikula01

Thu, 07/18/2013 - 17:56

The duration uncertainty—defined as pessimistic minus optimistic estimate - that can clear confusion which is in the question

cnppmp1's picture

cnppmp1

Thu, 07/18/2013 - 14:58

The information provided on PERT and uncertainty I think should be ignored and just look at the activity duration for the two activities mentioend and addup, 24+18=42

 

Regards

CN Patil

pmpnewster

Thu, 07/18/2013 - 15:18

 

Formula: Std Devn = Duration Uncertainty / 6

For 1st activity, StdDev1 = 18/6 = 3   ==> Variance1 = 3^2 = 9

For 2nd activity, StdDev2 = 24/6 = 4  ==> Variance2 = 4^2 = 16

 

Variance of critical path = Var1 + Var2 = 9 + 16 = 25

Std Dev of critical path = sqrt(25) = 5

==> Duration Uncertainty of critical path = 6*StdDev = 6*5 = 30 days

 

Choice 2: 30 days

 

ppalkonda1975

Thu, 07/18/2013 - 15:49

 choice 2 is correct answer

but how do we know we need to calculate std dev - duration uncertainty/6? I thought this calcuation was already done i.e. deviation = p-o/6 and then we have the values listed in the question....

where can i read more on these types of topics?

ppalkonda1975

Thu, 07/18/2013 - 15:53

 As i read more into your answer i udnerstand better

first we need to get

1. the std deviation

2. then variance and add variance (addition of variance cannot be the answer, you need to calculate back again the std deviation

3. calculate std deviation...

only the last part i need clarification for where you said Duration "Uncertainty of critical path = 6*StdDev = 6*5 = 30 days". ??

 

pmpnewster

Thu, 07/18/2013 - 16:26

 

I have used the same formula as in my earlier post:   Std Devn = Duration Uncertainty / 6

==> Duration Uncertainty = 6*Std Devn

Does that answer your question?

 

Regards.

 

ppalkonda1975

Thu, 07/18/2013 - 16:35

 yup it does..straight forward...but too much thinking can screw up ....thanks!!