# Help - Oliver Lehmann's 75 questions explantion

Submitted by rebeccafred on Sun, 04/22/2012 - 00:57

Hi,

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?

http://www.oliverlehmann.com/pmp-self-test/75-free-questions.htm

Thanks in advance.

Rebecca.

trawal

Sun, 04/22/2012 - 01:17

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## Add variances

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

Tue, 04/24/2012 - 19:21

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## Rebecca,the formula is that

Rebecca,

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?

projmanpro

Tue, 04/24/2012 - 19:48

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## Probability question

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

pmalik

Mon, 06/12/2017 - 08:32

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## For complete explanation on

For complete explanation on PERT and SD, you can read following articles

* PERT Formula and Use of Standard Deviation - http://www.pmbypm.com/pert-and-standard-deviation/

* How to use PERT, CPM and Standard Deviation Together? - http://www.pmbypm.com/critical-path-pert-and-standard-deviation/