PERT completion times revisited

Two sources of PERT completion time bias are well documented in the literature: near critical paths turning critical during execution and misspecified activity time probability models. Although simulation is clearly the most appropriate method for assessing project duration, most introductory discussions touch on these issues and move quickly to standard approximations, implying that PERT offers useful, if only approximate, project duration estimates. This paper uses simulation to illustrate the nature and extent of PERT approximation errors in simple examples from two excellent texts. The examples raise serious questions about the utility of PERT project duration estimates and suggest opportunities for improvement in introductory PERT instruction.