Question on PERT, distribution

Submitted by vijayavadrevu on Sat, 04/27/2013 - 06:03

 Assume the optimistic time estimate for a project is 16days, the pessimistic estimate is 34 days and the most likely estimate is 24 days. Using PERT, what is the range of this estimate and the type of distribution?

A) Ranges between 22 and 28 days with a bell distribution

B)Ranges between 22 and 28 days with a normal distribution

C)Ranges between 22 and 28 days with a uniform  distribution

D)Ranges between 22 and 28 days with a beta distribution

Answer is given as B

Regarding range, I am clear. How to determine the type of distribution? Can someone explain?

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sspawar

Sat, 04/27/2013 - 08:28

 I am not able to get what is it?

Can you tell what do you know and how do you know ranges are 22 and 28 with the given figures?

with 16,24,34

Ex time (Mean) = 24.333

SD (sigma)  = 3

+/-3sigma = 24.333-9 = 15.333 and 24.3333+9 = 33.3333

thus ranges 15.3333 to 33.3333 for normal distribution

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but options are provided range 22 to 28 with different type of distributions.

These are in between values , and may be possible for Normal as  well as Beta diatributions.

Bell distribution is incorrect term, in statistic maths,  normal distribution curve is known as bell curve or bell shape curve.

Uniform distribution does have only one value , may ranges to any given stretch.

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 Discard such stuffs.

 

vijayavadrevu

Sat, 04/27/2013 - 08:45

 Sir,

 

I have calculated the PERT using the formula P+(4*M)+O / 6 =( 34 + (4*24) + 16)/6 = 146/6 = 24.3 days

Standard deviation SD = (P-O)/6 = (34-16)/6= 3 

estimate for this project can be taken as 25 +/- 3 = 22 to 28 days

This was my understanding. If I am wrong , please correct me

 

Regards

Vijaya

rwmv

Sat, 04/27/2013 - 10:32

 Since this is 3 point estimate,beta and triangular as both are useful for representing the three-point estimates established during interviews

There is no option of choosing triangular so I would go with beta

I calculated with PERT formula- 3 point estimate and the answer was 24.33

rwmv

Sat, 04/27/2013 - 10:32

 Since this is 3 point estimate,beta and triangular as both are useful for representing the three-point estimates established during interviews

There is no option of choosing triangular so I would go with beta

I calculated with PERT formula- 3 point estimate and the answer was 24.33

rwmv

Sat, 04/27/2013 - 10:54

 Here is the reason why?

You must know some basic statistics for the PMP Certification exam. All equations are based on a normal distribution. In a normal distribution, keep the following in mind:

  • 68.3% of the data points fall within one standard deviation

  • 95.5% of the data points fall within two standard deviations

  • 99.7% of the data points fall within three standard deviations

If you're looking at a normal curve and need a cumulative distribution, you should remember these values:

  • 0.15% of the data points fall between 0 and -3σ from the mean

  • 2.25% of the data points fall between 0 and -2σ from the mean

  • 16% of the data points fall between 0 and -1σ from the mean

  • 84% of the data points fall between 0 and +1σ from the mean

  • 97.75% of the data points fall between 0 and +2σ from the mean

  • 99.85% of the data points fall between 0 and +3σ from the mean--he above problem SD is 3  so its normal distribution

rwmv

Sat, 04/27/2013 - 10:54

 Here is the reason why?

You must know some basic statistics for the PMP Certification exam. All equations are based on a normal distribution. In a normal distribution, keep the following in mind:

  • 68.3% of the data points fall within one standard deviation

  • 95.5% of the data points fall within two standard deviations

  • 99.7% of the data points fall within three standard deviations

If you're looking at a normal curve and need a cumulative distribution, you should remember these values:

  • 0.15% of the data points fall between 0 and -3σ from the mean

  • 2.25% of the data points fall between 0 and -2σ from the mean

  • 16% of the data points fall between 0 and -1σ from the mean

  • 84% of the data points fall between 0 and +1σ from the mean

  • 97.75% of the data points fall between 0 and +2σ from the mean

  • 99.85% of the data points fall between 0 and +3σ from the mean--he above problem SD is 3  so its normal distribution

sspawar

Sat, 04/27/2013 - 12:04

 why it is assumed that estimate will be at +/- 1 SD.

22-28 is possible for all 3 distributions = normal, beta, uniform.

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Discard such question bank.