# How to calculate the Probability distiribution

Submitted by kmkan28 on Thu, 02/28/2013 - 14:25

Hi,

I have got this below question from one of the mock test.I am not able to get the calculation.

Given that completion time of a particular project has an expected value of 8 weeks and a standard deviation of two weeks ,and assuming that completion time is normally distributed,what is the probability of completing the project within 10 weeks

A.0.5

B.0.84

C.0.95

D.0.997

Please let me know the explanation

Forums:

sspawar

Thu, 02/28/2013 - 15:07

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## It is good question, and I

It is good question, and I think - it is little different , than so far asked.

There are mainly four pobability distibution curve - mentioned in PMBOK also.

question is saying - normally distributed -- means normal distribution curve

It is same like we say for

+/- 1 sigma = 68.26 % area means 0.6826 proability, in figure if you will see left side of the mean (middle pick) is half of this 68.26 - 34.13%, and right side of the mean is also half of this 68.26 = 34.13%. lfet side called - (minus ) one sigma , right side called +(plus) one sigma.

like

+/-2 sigma = 95.46 % area means 0.9546 probability

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

now come to the question

sigma = 2 weeks

10 weeks means 8+2 week = mean + right side one sigma

corrosponding area

Left side complete area of curve up to mean ,is half of full bell shape curve = 50%

Right side 1 sigma area = 34.13%

=> 50+34.13 = 84.13, hence total probability is 0.8413,

Answer is Brefer both figures of following thread:

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

kmkan28

Thu, 02/28/2013 - 16:22

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## It is awesome

Pawar,

Thanks for your reply with the explanation.Indeed i was not aware of this much details and since you come from Civil engineering experience it helped me a lot.I am continuously attending the mock test and now i am able to answer on quetions with the similar format.

I have one more question likely the same which will be posted after some time.

Thanks,

Mani

sspawar

Fri, 03/01/2013 - 01:32

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## You are right. Definitely

You are right.

Definitely it is easy to learn maths part for an Engineer.

What is your next question.

sunitaPMP

Fri, 03/01/2013 - 07:41

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## do we get such type of

do we get such type of questions in real exam?

kmkan28

Fri, 03/01/2013 - 10:10

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## This is also my question

Do we get such type of questions in the PMP exam.Definately it will cosume time for non-maths proficients and other non subject matter experts?

Mani

sspawar

Fri, 03/01/2013 - 11:33

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## I believe this is not a

I believe this is not a lengthy or time taking question.

If your concepts are good then it is a half minute question

See given, SD =2, Mean = 8 , where will stay 10.

it is very easy now when you have been understood the concept.

50 (mean) +34(one sigma rt side)

kmkan28

Fri, 03/01/2013 - 11:44

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## Pawar,Yes,understanding the

Pawar,

Yes,understanding the question is key point.I understood after you have given the explanation for this question.

Likely if you have any such questions in other probablity distribution (ex: beta..etc.) please share in this forum.

Mani

sspawar

Fri, 03/01/2013 - 17:30

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## here are 2

here are 2 questions

http://er-sspawar.blogspot.in/2012/12/15-chance-you-will-exceed-estimate...

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

sunitaPMP

Fri, 03/01/2013 - 19:16

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## Honestly am not clear about

Honestly am not clear about this concept.Can you tell em where can I check this?Any link?I do not understand why this bell shpe,what is 50 % etc...Can you pls send me soem link to get thos eidea?

I just know how to calculate SD but i can not map this to that bell shaped curve.

Tahnks in advance...

sunitaPMP

Fri, 03/01/2013 - 19:17

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## Hello Pawar jiHonestly am not

Hello Pawar ji

Honestly am not clear about this concept.Can you tell em where can I check this?Any link?I do not understand why this bell shpe,what is 50 % etc...Can you pls send me soem link to get thos eidea?

I just know how to calculate SD but i can not map this to that bell shaped curve.

Tahnks in advance...

sspawar

Sat, 03/02/2013 - 01:23

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## you can refer youtubes on

you can refer youtubes on normal distribution curve and web sites mathisfun, wolform etc

for 50% remember NMD curve have mean value (mue) in middle , and either side of it have equal space 50-50 below curve based on x axis.

remember

6sigma = 99.99966/2 = 49.99999% not 50%

it is infinite curve.

sspawar

Sun, 03/03/2013 - 13:50

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## Sunita ji you want to know how

Sunita ji

you want to know how bell shape curve formed?

I try my best to explain this:

for example you are checking1000 tests given by pmp aspirants.

your standard value of profeciency is suppose 150 out of 200

this standard value 150 will be called as MEAN value.

now you observe that --- some students are performing better than this , some are below this.

then you make a table of marks at X axis against frequency of students obtaining that marks at Y axis

Values = Marks at X axis

Frequency =Nos of students at Y axis

100

30

110

45

120

100

130

120

140

135

150

150

160

130

170

125

180

90

190

45

200

30

Total

1000

sspawar

Sun, 03/03/2013 - 13:51

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## If it comes in following

If it comes in following shape --- it will be called as normally distributed.

µ

MEAN