Name E-mail Your e-mail address will never be displayed on the site. Subject Message [quote="Brandon A" post=14611]Hi All, I've been working through some sample questions over at Oliver Lehman's PM site. I came a across #27 in his 100 free PMP questions. I come from a quality background where we determine standard deviation empirically from a data set so these equations to approximate variance and SD are working a part of my brain that must be dormant. How do we arrive at 5.2 from the data set given? Any help is appreciated, see the problem below. Thanks! A project manager made 3-point estimates on a critical path along a sequence of activities A through E and found the following results: [img alt=data table]http://www.oliverlehmann.com/pmp-self-test/75-fre1.gif[/img] Assuming ±3 sigma precision level for each estimate, what is the calculated standard deviation of the allover path? App. 4.2 days [color=green]App. 5.2 days[/color] App. 6.2 days You can not derive the path standard deviation from the information given.[/quote]

## Topic History of : PERT/Standard Deviation Question

Max. showing the last 6 posts - (Last post first)
1 month 6 days ago #25335

Amit Kumar

Thank Brandon...was searching for this.
1 month 3 weeks ago #24876

Anonymous

in the question it state +/- 3 sigma
4 months 5 days ago #23540

Anonymous

I couldn't let it go either. Thank you.
4 months 3 weeks ago #23294

Anonymous

Thank you! It seems like the PERT weighted values in the table are included to throw us off!
2 years 6 months ago #14679

Anonymous

Thanks for clarifying
2 years 6 months ago #14677

Brandon A

Me again,

I found a similar problem and explanation which helped me figure out this one.

You must first calculate the standard deviations of each sequence activity, A through E, using (P-O)/6. Square each standard deviation to yield the variance (Variance = SD^2). Add all the sequence activity variances together then take the square root of the sum. This will yield 5.19... which rounds to 5.2. You could then express the schedule as 83.1 +/- 5.2 days.

I just passed the PMP today but couldn't let this question go.

Thanks

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