standard deviation question

ou have been recently assigned to provide an accurate project


schedule for a large construction company.


Your lead engineer has given you the following estimates for several


critical path activities. Your stakeholders want you to commit to a


completion date for these critical path activities. Which of the following


statements are true?


 









































Activity


OPT


PESSI


M.LIKELY


EXPT VAL


STD DEV


SD SQUIRED


TASK A


5


18


10


10.5


2.17


4.69


TASK B


28


61


40


41.5


5.50


30.25


TASK C


10


33


18


19.17


3.83


14.69


TOTAL


71.17


49.64



 


A. There is approx. 68 percent chance that the project will complete


between 64 days and 78 days.


B. There is approx. 95 percent chance that the project will complete


between 64 days and 78 days.


C. There is approx. 68 percent chance that the project will complete


between 57 days and 85 days.


D. There is approx. 99 percent chance that the project will complete


between 64 days and 78 days.


 


 


Answer: A


 


Please explain why/how the answer is "A"

According to the PERT calculation, the expected time to complete the task A, B and C is 71.7 as mentioned in the table. Now standard deviation for all the tasks together will be square root of 49.64 which will be 7 decimal something. So, The probablity that the actual project completion time will be between +SD and -SD , that is, 64 ans 78 days, would be 68%. Hope you got it .

Thanks for your response..really appreciate..

Can somebody explain?

 I got everything up until the 68% part... How do you calculate range of 64-71 to be 68% 

 I found this logic on another question:

 

http://pmzilla.com/how-calculate-approximate-probability-pmp

 

Still confused

Hi

what is chance in %age that work will be completed in between  57 days to 85 days.

regards

 

 

 

  There is approx. 95% percent chance that the project will complete

between 57 days and 85 days.

good

 

Pawar,

Can you please explain on how the answer 95% was derived ?

Thanks

Jai

Neha's explanation is correct, though not detailed. Some reactions are also correct, simply conveys the idea that you guys understand the subject. However, i am sure with the posted explanations, those who are begineers, probably they cannot still understand the subject.

Actually, the subject is about "PROBABILITY" the title of the post must be "Probability Question" and not standard deviation though SD is one of the variables (in addition to PERT estimate) used to calculate the probability. More so, there is a mistake in the data in the table - 71.17 is not the total of SD but the total weighted average mean (PERT estimate).

I want to share with you guys a clear explanation and a technique on how to calculate the probability based on PERT estimate and SD (normal distribution)

To have at least a background knowhow of the subject, let us define the following:
 
Standard deviation is a statistical concept that gives a measure of the 'spread' of the values of a random variable around the mean of a distribution. It shows how much variation there is from the average of the mean value. The more the variation, the more the uncertainty or risk in the process.
 
PERT assumes that the activity duration estimate follows the normal distribution. The normal distribution curve is a bell-shaped curve and is symmetrical on both sides of the mean. PERT normal distribution is best illustrated by the image below: (i cannot attach here, follow the link instead.)

http://www.pmptrend.com/index.php/en/forum/11-Questions-Answers-Queries-Infos-Etc/59-Probability-Calculation-Technique#59  

Hi Peojmanpro,

I cannot access the link you have provided. It says I do not have access. Can you please e-mail me the document to Jgrind07@gmail.com?

Appreciate you help in advance.

Thanks

Jai

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PMP Fighter's picture

base on you username, you are from china, please don't make any spam.

I am chinese too, but i come from Hong Kong, i am shame on you.

Cumilative Duration for the above
P-112, O-43, ML-68. So the duration for all 3 task is 71.17 and SD is 11.5 in that case.
1SD should be

+ or - 1 sd around mean is (71.17-(1*11.5)) and (71.17+(1*11.5)) or 59.67 to 82.67 days

please clarify. How come 7.04 is the 1SD.

There is a separate method to find out SD for the cumulative duration.

Find the SD for individual tasks.Square them to get individual variances.Add variances to get the cumulative variance.Then find its square root to get the SD for the cumulative duration.Hope it helps