TOPIC: PERT/Standard Deviation Question

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:



Assuming ±3 sigma precision level for each estimate, what is the calculated standard deviation of the allover path?

App. 4.2 days
App. 5.2 days
App. 6.2 days
You can not derive the path standard deviation from the information given.

hi,
there will no influence of the activity ,if we choose activity B,C and D
so calculate the SD for A and E =((35-17)/6)+((24-12)/6)=3+2=5
Pl correct me if I am wrong

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

Thanks for clarifying