PERT formula question - Need help
Submitted by Anonymous (not verified) on Sat, 07/07/2012 - 03:37
Can some one please help me this? Here is the question:
Together with your team, you applied three-point estimation on a critical path with constist of two activites.
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 formual for the 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.
Forums:
diba_perfect
Sat, 07/07/2012 - 04:08
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Here it's given that:Duration
Here it's given that:
Duration uncertainty(DU) = Pessimistic est.(P) - Optimistic est.(O); DU1 = 18 and DU2 = 24;
[and we know σ(standard deviation) = (P - O) / 6, so we can also say that σ = DU / 6 ------ (A)]
So for first activity, σ1 = 18 / 6 = 3{using (A)}; variance1 = 3^2 = 9;
Similarly, σ2 = 24 / 6 = 4; variance2 = 4^2 = 16;
Now, since these two activities are on the critical path, their variances can be added. So path variance = variance1 + variance2 = 9 + 16 = 25;
and σ of the entire path = sq. rt.(path variance) = sq. rt. (25) = 5
Again using (A), DU of the entire path = σ(path) * 6 = 5*6 = 30
So correct option is B.
Am I correct jcrajput?
sspawar
Sat, 07/07/2012 - 04:11
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it could be understood like that
In 1st Activity
Suppose Pess duration is = X days
Difference is 18 days , hence opt duration will be = X -18 days
In second actvity
Suppose Pess dur = Y days
Diff = 24 days , hence opt du = Y-24 days
X+Y will be pess dur for both activities
where as Opt dur will be = (X-18 +Y-24) days
Now Total uncertainity diff of combined package = P-O = X+Y - (X-18 + Y -24)
= 42 days
diba_perfect
Sat, 07/07/2012 - 05:46
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What's the correct option
What's the correct option Rajput?
jcrajput (not verified)
Sat, 07/07/2012 - 05:54
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The correct answer is B. That
The correct answer is B. That is 30 days.
This is question # 94 from Oliver F. Lehmann 175 questions.
Thank you for explanation.
krantikumar50
Sat, 07/07/2012 - 06:19
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WOW!!!!
Great Diba. Good explanation as well. If I could understand it, anyone else will. All the best Man!!! Enjoy! KK....
ameenhaqqi
Sat, 07/07/2012 - 06:26
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Here is another way to solve
Here is another way to solve it:
Standard Deviation (pessimistic minus optimistic estimate) is given for 2 activities
Activity 1 Standard Deviation = 18 days
Activity 2 Standard Deviation = 24 days
Variance for Activity 1 = 18x18 = 324
Variance for Activity 2 = 24x24 = 576
Total Variance on critical path = 324+576 = 900
SD for the entire path = Square Root of 900
SD for the entire path = 30 days
diba_perfect
Sat, 07/07/2012 - 07:43
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Ameen,Standard deviation
Ameen,
Standard deviation isn't = (pessimistic est. - optimistic est.) .. it's rather = (pessimistic est. - optimistic est.)/6 ..
I would say that your solution is arithmetically correct(since you've ignored the division by 6 all throughout, the answer is correct) but technically incorrect.
ameenhaqqi
Sat, 07/07/2012 - 10:13
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Diba, your method to solve
Diba, your method to solve this question is absolutely correct; however, both methods give you exactly the same answer with any combination you use. Thanks for the correction :)
sspawar
Sat, 07/07/2012 - 13:53
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thanks everybody
Thanks to correct me
only the correction in Diba solution is 18 and 24 and 30 is not Du but Diff.
This question is based on or like of q 27 of O L 75 q.
Regards