Computational complexity of PERT problems

Computational complexity results for two PERT problems are presented. A project is specified by precedence relations among tasks. Task durations are independent random variables with discrete, finite ranges. The following results are obtained: (1) computing a value of the cumulative distribution function of project duration is #P-complete, (2) computing the mean of the distribution is at least as hard, and (3) neither of the problems in (1) and (2) can be computed in time polynomial in the number of points in the range of the project duration unless P = NP.