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TOPIC: Does any one have any idea how to approach this PERT question?

Does any one have any idea how to approach this PERT question? 1 week 11 hours ago #22623

  • Ali MOMENZADEHPISHROO
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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 (beta distribution), what is the duration
uncertainty of the entire path?

o 21 days
o 30 days
o 42 days
o No statement is possible from the information given
FROM OLIVER LEMANN FREE SAMPLES
Last edit: by Ali MOMENZADEHPISHROO.

Does any one have any idea how to approach this PERT question? 1 week 7 hours ago #22632

  • Gabriella Dellino, PMP
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Wow, Ali! That truly is a tough one!

Let me recap the assumptions here:
1. Two activities on the critical path
2. Activity duration following a Beta distribution
3. Duration uncertainty (i.e., standard deviation, sigma) known for each activity.
Please notice that the description here sounds a bit misleading to me. In fact, if you assume a beta distribution, the standard deviation is sigma = (P - O)/6, where P and O are the most pessimistic and most optimistic durations, respectively. Not that we will use this formula to solve this problem, but reading it as P - O confused me in the beginning.
4. Confidence interval as +/- 3sigma. This won't be needed to solve our problem ;)

Now, from some statistical theorems, we know that the variance of the project is equal to the sum of the variances of the activities on the critical path. In our case:
var_p = var_1 + var_2 = (sigma_1)^2 + (sigma_2)^2 = 18^2 + 24^2 = 900
So the standard deviation for the project (i.e., its duration uncertainty) is sigma_p = sqrt(var_p) = 30.

Therefore the correct answer should be option B.

That said, I've never seen anything like this in the actual exam, nor even in the exam simulations I practiced. Thanks for sharing it.

Gabriella
Gabriella Dellino, PhD, PMP
PMPrepCast Community Moderator

Does any one have any idea how to approach this PERT question? 6 days 10 hours ago #22649

  • Ali MOMENZADEHPISHROO
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Awesome !Good job
Thanks very much Gabriella
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