Help - Oliver Lehmann's 75 questions explantion



I attempted the Oliver Lehmanns's 75 Qs. I am stumped by Q 27 on the Std Deviation Calculation.


Can someone post an explanation please?


Thanks in advance.


You need to calculate variances of all 4 cases and then take sqr root of that to get the std variance.


I hope this would help

Oliver F. Lehmann's picture


the formula is that the path standard deviation is the square root of the summed up squares of the single SDs. 

A single SD (= sigma) is defined as a 6th (±3 Sigmas) of Pess. minus Opt.

Single SDs are: Act. A: 2 days, Act. B: 1 day;  Act. C: 2 days, Act D: 3 days, Act. E: 3 days.

SD squares are therefore:  Act. A: 4; Act. B: 1;  Act. C: 4; Act D: 9; Act. E: 9 

SD squares, sum: 27

Square root of that: 5.19 days

This means that the distance between a pessimistic and an optimistic estimate for the entire path duration should be 6 x 5.2 = 31.2 days.

It is one of my toughest questions, and I hope that you won't see it in the exam, but who knows for sure?

 I have explained and detailed these kinds of question in my thread at forum with graphs u can visit that thread

For complete explanation on PERT and SD, you can read following articles
* PERT Formula and Use of Standard Deviation -
* How to use PERT, CPM and Standard Deviation Together? -

Help oliver lehmann's 75 questions explantion. This explanation is here and only necessary for all of those who want to define what they are asking in questions. Some of the questions are perfectly define on the page but some of these are not valid.