probability question - Please explain

Submitted by sai_murthy on Fri, 07/30/2010 - 05:04

Question:


Can you please explain me how the answer is 99.73%


An activity has an optimistic estimate of ten days, pessimistic estimate of sixteen days,
and most likely estimate of thirteen days. Assuming a normal distribution,
what is the probability that the task will be completed within 10 to 16 days?


99.99 %
99.73 %
95.46 %
68.26%


Ans : 99.73%

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jagjitdhaliwal@...

Fri, 07/30/2010 - 06:41

Hi Sai,

With P = 10, M = 13 and O=16, the EAD can be calculated to come as 13.

So, the deviation in duration 10 to 16 days from mean(13) is +/-3. Since it's 3-sigma, % of occurances to fall between two control limits is 99.73%.

Could you please share source of question as I think question is bit vague. Anyway, I assume above as reasoning for answer.

Regards,

Jagjit

 

 

sai_murthy

Fri, 07/30/2010 - 07:07

Thank you Jagjit.


For the explanation. Soruce is PMStudy.com

jagjitdhaliwal@...

Sat, 07/31/2010 - 18:17

Hi Sai,

Came across following link which I think explains the reasoning better than mine.

http://www.youtube.com/user/SirGanttalot

Regards,

Jagjit

 

 

sai_murthy

Sat, 07/31/2010 - 21:56

Jagjit


You are correct.


Mean = Most Likely = 13



In PERT we do probabilistic estimation ,



PERT Sigma (Standard Deviation) = (Pessimistic - Optimistic)/6 ......  See the formula given in Quality KA under Standard Deviation slide



= (16 - 10)/6  = 1



Probability to be 99.73% means 3 Sigma Control limit



UCL = Mean + 3*Sigma = 13 + 3*1 = 16
LCL = Mean - 3*Sigma = 13 - 3* 1 = 10