3 point estimates

A project manager made 3-point estimates on a critical path and found the following results:

Assuming ±3 sigma precision level for each estimate, what is the 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.

Can you pleaseexplain how?

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Some points to remember:-

  • 1 sigma is equal to 1 standard deviation, so, 3 sigma is equal to 3 standard deviations. This information will not be applied in the question.
  • Standard deviation can be negative or positive; so, in order to calculate the standard deviation of multiple tasks, don’t sum up the standard deviation of each task.
  • In order to calculate the standard deviation of more than one task (on critical path), calculate the standard deviation of each task, take a square of standard deviation of these tasks, sum up the square value, and at the end, calculate the square root of this summed value.
  • Standard deviation of each task is (p-o)/6
  • Variance is a square of Standard deviation.
  • Easy way to remember the formula between Standard Deviation and Variance is



Apply above mentioned points for calculating the SD of allover path. Calculate SD of individual task:-

  • SD of A = (p-o)/6= (24-12)/6= 2
  • SD of B = 6/6 =1
  • SD of C = 12/6 = 2
  • SD of D = 18/6 = 3
  • SD of E = 18/6 = 3

Calculate V of individual task:-

  • V of A= (SD of A)2 = 4
  • V of B = 1
  • V of C = 4
  • V of D = 9
  • V of E = 9

Calculate V of allover path:-

  • 4+1+4+9+9 = 27

Calculate SD of allover path:-

  • SD = Square root of V
  • So, calculate the square root of 27 which is 5.2
  • Answer is B

Let me know if you still have any confusion.

Thanks & Regards,

Hemant Tandon, PMP, CAPM

The question ask the Standard Deviation and we calculate the Sum of the Variation and we Square it,  


I dont understand why we talk about Standard deviation and play with Variation as it's two different thing


The standard deviation of the entire path is the square root of the variance of individual activity as rightly mentioned by Hemant.

This is different from the standard deviation of an individual activitiy.