Please advise on PERT questions with logical explanation.

Submitted by amber on Tue, 10/23/2012 - 05:52


16.


 



If you know the expected value is 500 and the standard deviation is 12, you can say with

A.

The activity will take from 488 to 512 days.

B.

The activity will take from 464 to 536 days.

C.

The activity will take from 494 to 506 days.

D.

The activity will take from 476 to 524 days.

17.


is true?

If your expected value is 110 and the standard deviation is 12, which of the following

A.

There is approximately a 99 percent chance of completing this activity in 86 to 134 days.

B.

There is approximately a 68 percent chance of completing this activity in 98 to 122 days.

C.

There is approximately a 95 percent chance of completing this activity in 98 to 122 days.

D.

There is approximately a 75 percent chance of completing this activity in 86 to 134 days.

Answe 16.


standard deviations. The expected value is 500, and the standard deviation times 2 is 24, so


the activity will take from 476 to 524 days.

D. There is a 95 percent probability that the work will finish within plus or minus two

Answer 17.


95 percent probability uses plus or minus two standard deviations, and a 99 percent probability


uses plus or minus three standard deviations.

B. A 68 percent probability is calculated using plus or minus one standard deviation, a

I am not able to understand the logic behind the answer of these questions. Pls explain?



approximately a 95 percent confidence rating which of the following?

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anand_kl

Tue, 10/23/2012 - 07:30

Hi,


Lets know the formulae first:


EAD = (O + 4M + P) / 6


SD = (P-O)/6


Variance = Square of (SD), which also means  SD = Square root (Variance)


Now as per standard practice and as per Rita Mulachy book as well, duration range is calculated with below process:


1) Calculate EAD of all the activities and get the sum of EAD to get Cumulative EAD


2) Calculate Variance of all the activities, sum the variance to get cumulative variance


3) Take the Square root cumulative variance to arrive at cumulative SD


4) Duration range  = Cumulative EAD +/- Cumulative SD


5) SD is also known as Sigma.



  • 1 sigma = 68 .26%

  • 2 Sigma = 95.46%

  • 3 Sigma = 99.73 %

  • 6 Sigma = 99.99%

This is the logic behind the calculations. You can apply these to find out the answers.


Regards,


Anand KL, PMP