Page 78 - The Six Sigma Project Planner

availability) this would mean that the project is expected to finish on May 8. A 95%
confidence interval (expected completion date ±2 sigma, or ±7 days) for schedule
completion would be from April 24 to May 23. Note that this interval is much tighter
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than that obtained in the best-case/worst-case analysis.
Probability of Meeting Project Due Date
With the information we now have, it is possible to calculate the probability that the
project will be completed on or before its due date. This is done by calculating the Z
statistic using the due date and the calculated schedule mean and standard deviation.
The calculations are shown below:

Z = Due date duration - Project mean duration (Equation 1)
Project sigma

For example, assume that the project due date is Friday, May 16 for a project due date
duration of 34 workdays. This gives

Z = 34 - 27.83 = 1.83
3.37

The area of a standard normal curve below z = 1.83 is 96.6%. Assuming that the
duration estimates are correct, this is the probability of meeting the project due date.

Note: The MS Excel function for the cumulative standard normal distribution is
=NORMDIST(Z,0,1,TRUE). In our example, z = 1.83, so the function becomes
=NORMDIST(1.83,0,1,TRUE).

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Note: If your project has multiple critical paths, or if the 95% confidence interval for a noncritical path
extends beyond the confidence interval for the critical path, project schedule estimates should be based
on the path that gives the longest completion time estimates.

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