A Review of Activity Time Distributions in Risk Analysis <s> 9) The Gamma Distribution <s> Summary ::: ::: ::: ::: This paper deals with an examination of the well-known Project Evaluation and Review Technique (PERT). Every activity duration is supposed to have a gamma distribution whose parameter values are determined from the optimistic, the most likely, and the pessimistic estimate of the duration. The expectation and the variance of the earliest event times are computed according to a method given by CHARLES E. CLARK in reference 2. ::: ::: ::: ::: One can then estimate the risk that a PERT planning, which is essentially a planning according to the critical path method, will be infeasible. ::: ::: ::: ::: During the examination it was found that errors in the pessimistic estimates may have serious consequences. <s> BIB001 </s> A Review of Activity Time Distributions in Risk Analysis <s> 9) The Gamma Distribution <s> The original PERT formulae [5] employ the mode and endpoints to estimate the mean and standard deviation of subjective probability distributions. Though widely-used, they have met with criticism for their inaccuracy, and for their being limited to the beta distribution when there is no a priori reason why the distribution should be beta ([4], [6]). This note presents alternative, accurate and relatively distribution-free formulae for estimating the means and standard deviations of subjective probability distributions. <s> BIB002
Lootsma BIB001 examined PERT and proposed a model for a project which every activity time follows a gamma distribution with density  also used the gamma distribution as an activity time distribution. His method followed after SANs. See Perry and Greig BIB002 for more on gamma activity time distribution.