Project probabilities

Posted on 22nd September, by JimYoung in Blog. No Comments

We don’t need expensive software to answer questions such as:

  1. When do we reasonably expect to complete our project?
  2. What’s the probability of completing our project on time or on a given date?
  3. Which tasks on the critical path should we focus on to meet the schedule?
  4. With a 95 percent confidence level how much will our project cost?
  5. How much variance is associated with the total man-hours we’ve estimated for this project?

Well, here’s a worked example.  We need a calculator with a square root function.  Given the task durations below, what’s the probability of completing this project in 13 days?

picture 1

First we calculate variance and standard deviation along the
critical path:


Based the probability distribution table, the Z Score of 0.46 provides a probability of 0.1772, and since 13 is less than BET we subtract 0.1772 from 0.5.  The resultant 0.3228 is the probability (ie, 32%) that the project will be completed in 13 days.  Easy eh?  But like all such estimates, it’s only as good as the input data.

So now if our client asks “What’s the likelihood of the project being completed by Xmas?” we know to run a mile!

Probability Distribution Table 


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