There is an extensive body of literature describing the serious flaws in the use of PERT, or at least in the "belief" in the PERT numbers.
http://herdingcats.typepad.com/my_weblog/2007/06/pert-analysis-r.html is a short summary of this topic.
While the PERT formula may stimulate discussion about the variances in the completion dates or durations of a project, it should not be used without serious consideration of its 20% to 50% unfavorable bias - optimistic bias - on anything but the most trivial activity networks.
DID 81650 - the Federal guideline for the construction of the Integrated Master Schedule - requires a schedule risk analysis ?2.4.1.23 using statistical methods. The standard in today?s project management practice is the Monte Carlo Simulation that removes the variances and errors of PERT.
