Why Projects Are “Always” Late: A Rationale Based on Manual Simulation of a PERT/CPM Network

Richard J. Schonberger
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Richard J. Schonberger: Department of Management, University of Nebraska---Lincoln, Lincoln, Nebraska 68588

Interfaces, 1981, vol. 11, issue 5, 66-70

Abstract: The conventional procedure for determining the expected project completion time is to find the critical path; its duration is treated as the expected completion time for the project. It is known among the community of system simulation scholars, however, that deterministic critical path analysis yields certain conclusions that may be disproven via Monte-Carlo simulation of a project network. (Klingel observed this in 1966.) Although the body of literature on network simulation has grown a good deal in recent years, the concepts generally have not found a place in the decision processes of practicing project managers. In view of the erroneous expectations that deterministic network analysis leads to, there seems to be a need to disseminate network simulation truths more widely and effectively. The approach taken in this paper is to explain the phenomenon of projects “always” being late by proving, in the simplest way possible, that the deterministic critical path understates the likely project duration. The proof is developed via a manual method of simulating a simple network using probabilistic activity times. Some generalizations for improved project management, based on network simulation based concepts, are offered in the final portion of the paper.

Keywords: networks; project management: PERT/CPM; simulation (search for similar items in EconPapers)
Date: 1981
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