PERT/SD Question

Submitted by Julbrown31 on Tue, 04/16/2013 - 03:34

 






















27. A project manager made 3-point estimates on a critical path and found the following results:

Assuming ±3 sigma precision level for each estimate, what is the standard deviation of the allover path?
 
  App. 4.2 days
  App. 5.2 days
  App. 6.2 days
  You can not derive the path standard deviation from the information given.

 


 


 


I was unable to come up with any of the answers.. I was under the impression that allover means add the total's of both Pessimistic and Optimistic to plug into the SD calculation


 


128-62 /6 = 11... What is wrong with my logic?

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sspawar

Tue, 04/16/2013 - 06:16

 refer this site :http://www.mathsisfun.com/data/standard-deviation.html

Now, we calculate each dogs difference from the Mean:

To calculate the Variance, take each difference, square it, and then average the result:

So, the Variance is 21,704.

And the Standard Deviation is just the square root of Variance, so:

Standard Deviation: σ = √21,704 = 147.32... = 147 (to the nearest mm)

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you cant add directly all optimistics and passamistics, because o/p of each activity may overlap on both side activities.

by definition sd is average of spread value of all data from mean, and thus half value comes in minus and half in plus, thus summation could not be correct value.

thats why first, variances(square of differences) of each data is added and then taken  rootsquared for SD.

y0zh

Tue, 04/16/2013 - 08:02

based on your calculation


pessimistic variance = 10.64


optimistic variance = 46.64


 


how to get SD now for both pessimistic and optimistic? I got 6...is that correct?

felice821

Tue, 04/16/2013 - 08:12

Standard Deviation

Act A = 24 - 12 / 6 = 2. 

Act B = 14 - 8 / 6 =1

Act C = 27 - 15 / 6 = 2

Act D = 28 - 10 / 6 = 3

Act E = 35 - 17 / 6 = 3

 

Standard Deviation Square

Act A = 4

Act B = 1

Act C = 4

Act D = 9

Act E = 9

 

Square Root of SD Square

27 square root = 5.19

 

Answer should be App. 5.2 days

y0zh

Tue, 04/16/2013 - 08:16

Thank you

Julbrown31

Wed, 04/17/2013 - 03:54

 Thanks... I really need to nail these on the exam.