Can't see why Option B is correct. pls help
Submitted by Anonymous (not verified) on Wed, 11/28/2007 - 21:44
The following three tasks form the entire critical path of the project network. The three estimates of each of these tasks are tabulated below. How long would the project take to complete expressed with an accuracy of one standard deviation?
| Task | Optimistic | Most likely | Pessimistic |
|---|---|---|---|
| A | 15 | 25 | 47 |
| B | 12 | 22 | 35 |
| C | 16 | 27 | 32 |
A. 75.5
B. 75.5 +/- 5.08
C. 75.5 +/- 8.5
D. 75.5 +/- 2.83
Forums:
eugen (not verified)
Mon, 01/07/2008 - 16:46
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auto cleansed :)
auto cleansed
(there is no button for delete)
anojv3 (not verified)
Tue, 06/10/2008 - 13:43
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Can't see why Option B is correct. pls help
the option B is correct but the values is 75.5 + - 7.08 [(sqrt( sum of standard deviation of all the three tasks)) = 7.08] and not 5.08
vaalo (not verified)
Fri, 06/27/2008 - 22:49
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probability
the answer couldn't be 75.5+ -7.08 because the sum of the most likely estmates is 74, therefore it should be 74 + - 7.08
mlleeder
Wed, 08/06/2008 - 02:48
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Probability calculation
75.5 looks right based upon the calculation for PERT:
(P + O + (4*M))/6
(114 + 43 + (4 * 74))/6 = 75.5
But....
Regarding the post above with the standard deviation calculation:
I'm a bit confused by this - isn't the "standard deviation" calculation for this type of question/problem: (P-O)/6 ???
I have this listed in different study literature (one which I downloaded from this site).
Granted, it doesn't fit this answer, but where does it fit in?
Should I be prepared to use the more traditional standard deviation formula?
..just wondering....Thanks!
hodgiemodough
Wed, 11/18/2009 - 20:18
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It's not a good question if the answers are wrong!
This question is throwing you off because all the answers are wrong… well at least the second part is.
Check it out:
Solve PERT (O+4M+P) / 6 right? So Tasks A = 27, B = 22.5, and C = 26. That makes the total estimated time on the critical path = 75.5 and since all the answers have 75.5 we know we are on the right track.
Now solve the variance (P-O / 6)2 Tasks A=28.44, B=14.69, and C=7.11 Total variance is 50.24
Now solve the square root of 50.24 and since 7x7=49, it has to be an answer with + 7, but look there aren’t any answers with + 7 …hum did I do something wrong?
It can’t be the 5 right? 5x5=25. It can’t be the 8 and it's certainly not the 2.
The answer is E. 75.5 + 7.09
The answers are flawed. Am I right?
spatel0169
Fri, 11/20/2009 - 14:02
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It's a typo. Replace 47 with
It's a typo. Replace 47 with 27 and everything will fall in place.
hodgiemodough
Fri, 11/20/2009 - 15:49
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Yeah or just replace the
Yeah or just replace the +5.08 with +7.09 and everything will fall into place as well.
Using the information provided in the question (including the typo) are my calculations correct?
Mike Jones
Wed, 12/02/2009 - 15:39
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Mike Jones
Wed, 12/02/2009 - 15:35
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Further to these comments
agree fully with the answers below (75.5 + 7.09) ... just thought this a good place to link to this pdp that explains pert and the degrees of confidence really well. You can throw an excel spreadsheet table together in a few seconds that will give you the PERT and confidence levels even up above single sigma level.
http://www.pendere.com/publications/Using%20PERT%20for%20Schedule%20Estimation.pdf
hodgiemodough
Wed, 12/02/2009 - 15:48
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Thanks Mike. I'll certainly
Thanks Mike. I'll certainly read it.