## On Standard Deviations

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tecklung
Posts: 12
Joined: Tue Sep 30, 2014 6:36 pm

### On Standard Deviations

Dear all,

I would like to deduce standard deviations and their associated % and make a list to memorize from the below question from a Mock Exam.

Can I say that...
1 Standard Deviation = 68%
2 Standard Deviations = 95%
3 Standard Deviations = 99%

Do correct me if I am wrong. Thanks.

QUESTION
If you are given that the expected value for your project duration estimate is 627 days, and the most likely duration estimate is 575. The standard deviation is 14. Which of the following statements is true?

A. You have 68 percent confidence that the project duration is between 547 days and 603 days.
B. You have 95 percent confidence that the project duration is between 547 days and 603 days.
C. You have 95 percent confidence that the project duration is between 613 days and 641 days.
D. You have 99 percent confidence that the project duration is between 585 days and 669 days.

99 percent confidence level is determined by adding and subtracting three standard deviations to the expected value. Therefore, the range of dates for this question is 585 days to 669 days.
bs_pani
Super Member Posts: 663
Joined: Mon Sep 22, 2014 3:36 pm

### Re: On Standard Deviations

I will go with option C.

EAD (Expected Activities Duration) or = 627 days
SD (Standard Deviation) = +/- 14 days
So the Optimistic Value is = 627 = 14 = 613. And Pessimistic value is = 627 + 14 = 641

But again I am not sure about the value stated for 1,2,3 standard deviation as 68%, 95% and 98%

Again I am also not sure about “99 percent confidence level is determined by adding and subtracting three standard deviations to the expected value” statement
bs_pani
Super Member Posts: 663
Joined: Mon Sep 22, 2014 3:36 pm

### Re: On Standard Deviations

Here is a old tread for the same kind of discussion..

viewtopic.php?f=11&t=371
bs_pani
Super Member Posts: 663
Joined: Mon Sep 22, 2014 3:36 pm

### Re: On Standard Deviations

After going through the above blog answer should be D.

But where is the ref for calculating SD for 65%, 95% and 99%.

Can someone plz point me where to look for ?
signor_rk
Contributor Posts: 182
Joined: Wed Jun 18, 2014 3:47 pm

### Re: On Standard Deviations

Option # D. You have 99 percent confidence that the project duration is between 585 days and 669 days.
Explanation :
For 99% confidence level , the project duration range is: (627 - 3*14) days and (627+3*14) days , i.e., 585 days and 669 days. Other Options are incorrect.

So far to my knowledge , we need to memorize the followings approx. values as stated by Teck for PMP Exam:
1 Standard Deviation = 68%
2 Standard Deviations = 95%
3 Standard Deviations = 99%

But still if you wish to go more into details , then here is an useful link:
http://en.wikipedia.org/wiki/Standard_deviation

Thanks
Regards ,

Ranjit
tecklung
Posts: 12
Joined: Tue Sep 30, 2014 6:36 pm

### Re: On Standard Deviations

Thanks all for the help. I manage to understand the concept.