PERT calculation

Submitted by ksee00 on Sat, 01/15/2011 - 00:19

Hi,


 


can anyone help with this question?


A project manager has been asked by the client to meet the promise date of the project. The project manager analyzes the schedule before promising a date to the customer. The project manager uses the program evaluation and review technique to evaluate the project schedule. She decides that based on the PERT calculations she can promise a delivery date of June 30. The expected value of the project completion date is May 30. If the project manager is willing to accept a 5% probability that the project will be delivered later than June 30, what is the standard deviation of the duration of the activities on the critical path? Assume a five-day workweek.

a. Ten days
b. Fifteen days
c. One-half month
d. One month

answer provided: c


 


Thanks


 

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gwashngo

Mon, 01/17/2011 - 12:53

Author of the question probably assumes that Delivery date is distributed normally around Expected Value of the project completion date (in this case 30th of May). The maximum acceptable delay is 31 days or 1 month.

According to table:

http://en.wikipedia.org/wiki/Standard_deviation#Rules_for_normally_distributed_data

to have 95% of probability of completing the project not later than June 30:

 1.96 * std_dev  <= 31 days (1 month)

what gives:

std_dev <= 15,81 days or 0,51 month.

Why the correct answer is c - I don't know, probably is only 2% deviated from "One-half month" when 15,81 is deviated 5% from "Fifteen days".

Five-day workweek has nothing to do with the question.

 

 

 

niteshrs

Wed, 05/25/2011 - 11:38

any comments...what is the right answer plz?

aaron

Wed, 05/25/2011 - 19:48

5% deviation means there is 95% probability that project will be delivered on time which means we are looking at 2 SD variation.

2 SD = 1 month

1 Stanard Deviation = 0.5 month

Answer B (15 days) could have been right answer if the question was described in days  (30 days) instead of month names (30 May - 30 Jun) . In current situation 0.5 month is the correct.