# Standard Deviation

Can somebody explain the following question please?

**A project manager has been asked by the client to meet the promise date of the project. The project manager analyzes the schedule before promising a date to the customer. The project manager uses the program evaluation and review technique to evaluate the project schedule. She decides that based on the PERT calculations she can promise a delivery date of June 30. The expected value of the project completion date is May 30. If the project manager is willing to accept a 5% probability that the project will be delivered later than June 30, what is the standard deviation of the duration of the activities on the critical path? Assume a five-day workweek.**

a. Ten days

b. Fifteen days

c. One-half month

d. One month

sspawar

Fri, 08/03/2012 - 15:29

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## As per 3point estimate

As per 3point estimate (PERT - wieghted average beta distribution method)

ans will be 15 days

refer earlier Question

http://www.pmzilla.com/mock-questions

http://er-sspawar.blogspot.in/

MuhammadKhan74

Fri, 08/03/2012 - 16:05

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## Thanks a lot. Regards, MK

Thanks a lot.

Regards,

MK

sspawar

Sun, 08/05/2012 - 03:12

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## THIS ARTICLE MAY HELP A LOT

THIS ARTICLE MAY HELP A LOT -from -http://www.interventions.org/pertcpm.html## 5.2. Probability of Project Completion by Due Date

Now, although the project is estimated to be completed within 28 weeks (t

_{e}=28) our Project Director would like to know what is the probability that the project might be completed within 25 weeks (i.e. Due Date or D=25).For this calculation, we use the formula for calculating Z, the number of standard deviations that D is away from t

_{e}.By looking at the following extract from a standard normal table, we see that the probability associated with a Z of -0.6 is 0.274. This means that the chance of the project being completed within 25 weeks, instead of the expected 28 weeks is about 2 out of 7. Not very encouraging.

On the other hand, the probability that the project will be completed within 33 weeks is calculated as follows:

The probability associated with Z= +1 is 0.84134. This is a strong probability, and indicates that the odds are 16 to 3 that the project will be completed by the due date.

If the probability of an event is p, the odds for its occurrence are a to b, where:

## Select Bibliography

sspawar

Sun, 08/05/2012 - 13:06

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## Most of the places I saw that

Most of the places I saw that this question is wrongly answered as option C =45 days(one and half month), without any justification.

I saw a blog in PMHUB, and many other sites , all are just copying , as 45 days without knowing the reasons why.

Some of them are little adding it with 95 % probability (appx) of 2sigma and thus of normal distribution but still they are answering 1.50month.

If It is by 2sigma of NDC, then one sigma = 31/2= 15days appx, also answer comes B. But it is wrong approach.

at here 95 % is different then

what

shown in NDC figure

see 47.87 (LHS)+47.87(RHS) = 95.73 %

where as in question it is

50(LHS) + 45 (RHS) = 95%

it makes a great difference

47.87-45 = 2.87%

with this difference at ends (flattered curve) = 45will give 1.65SIGMA, (value of Z, in Z score curve), while 47.87 will give = 2sigma.

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It can be checked in Excel program also

There is one formula NORMINV ( prob %age in decimal, MEAN DATE, SD) will give END date (pessimestic date).

But in this question given M = a Mean date, Prob %age = 95% and End date and asking SD =?

Could not be solved , directly.

But by indirect , by GOAL seek function of Excel =

Putting in formula NORMINV (0.95,30th May,SD) = 30th of June, by goal seek will give SD = 18.80 days( it also works on Normal Distribution Curve theory)

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BY PERT formula SD = P-O/6, Beta distribution , weighted average, method

Assuming -3sigma as Optimistic values = 3*31/1.645= 54 days, and pessimistic as cutoff date at 95% prob = 31 days from mean

= p-o/6 = 85/6 =14.1 appx = 15 days.

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By no other way answer will come.

ken222

Fri, 11/25/2016 - 11:51

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pmalik

Sat, 12/03/2016 - 12:09

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## PERT & SD

The ans would be 15 days.

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?