PERT Question - Need Help

Submitted by Nader on Tue, 09/18/2012 - 12:25

Dear All,


I have found an interesting question, but I'm facing some trouble getting an answer. Please check below:


Thanx in advance ...


The project manager for a project is going to use the PERT method of scheduling for the project that she is working on.


Using PERT calculation the variance for the project is found to be 25 days.


What is the range of values for the project duration such that here will be a least 95% probability that the actual project completion will fall between the high and low value of range of values:


A. 164 – 173 days
B. 144 – 194 days
C. 119 – 219 days
D. 159 – 179 days


 

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sspawar

Wed, 09/19/2012 - 05:12

Variance = 25

SD =          5

Around mean value 95%  (95.45%) probablity  +/- 47.52% have +/-2sigma

hence variation in between high and low value will be =   +/- 2*5 = 10 +10 =20

Hence option D. 159 – 179 days

Nader

Wed, 09/19/2012 - 08:25

Thank you sspawar... its clear now.


Have a good day,


Nader


Also I found this chart to be really useful, in our case we are interested in the 95.44% graph:


y17865

Thu, 09/20/2012 - 23:09

Would you illustrate with more details


 


SD=5 ?


 


etc

sspawar

Fri, 09/21/2012 - 01:21

its a well known PMP certified formula


SD(Sigma) = underroot of variance


or


V= Sigma^2 or SD^2


----------------------------------------------------------------


Moreover there is one formula


(P-O)/6 = SD or Sigma


here if you will try at


179-159/6   = it will not come= 5, because it is +/-2sigma ( 95.44%) difference


In this formula limit values are considered at  +/- 3sigma  (at 99.73% total probability with the mean value) and here it will be =2* 3*5 = 30


It means if  diffrence of P-O becomes 30 then SD = 30/6 =5.


and end value symmetrically will be  = 184 to154 at 99.73% probability.


 

sunitaPMP

Thu, 02/28/2013 - 04:19

hello pawar ji,

I did not understand this part

Around mean value 95%  (95.45%) probablity  +/- 47.52% have +/-2sigma

Can you please explain?

Thanks..

sspawar

Thu, 02/28/2013 - 14:39

 =95.45/2 = 47.72

 

2SIGMA AREA MEANS

EITHER SIDE OF MEAN (0, 0 ORIGIN) ,

+2SIGMA AND -2SIGMA

= (34.1+13.62)  + (34.1+13.62) =95.45%

 = (47.72)           + (47.72)              =95.45%

refer this thread - both figures

http://er-sspawar.blogspot.in/2012_05_29_archive.html

mercurie

Thu, 02/28/2013 - 08:31

 Please memorize the sigma values

6 sigma   99.99

3 sigma 99.73

2 sigma 95.46

1 sigma 68.26

If you are lucky, might even get direct questions

Please see the range given in each of the options

a. 9 days   (+ / -  4.5)
b. 50 days (+ / -  25)
c. 10 days  (+ / -   5)
d. 20 days  ( + / -10)
 
In this question, SD (sigma) is 5, and probablity is 2 sigma, that is +/- 10 
 
So the answer is straight away the option D
 
 
 
 
 
 
 
 
 
 

Alpha_romeo0

Thu, 02/28/2013 - 23:32

Heck l like your explanation best of all.. thanks!!!