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

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  • Richard J. Schonberger

    (Department of Management, University of Nebraska---Lincoln, Lincoln, Nebraska 68588)

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.

Suggested Citation

  • Richard J. Schonberger, 1981. "Why Projects Are “Always” Late: A Rationale Based on Manual Simulation of a PERT/CPM Network," Interfaces, INFORMS, vol. 11(5), pages 66-70, October.
    • Handle: RePEc:inm:orinte:v:11:y:1981:i:5:p:66-70
    DOI: 10.1287/inte.11.5.66
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    Citations

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    Cited by:

    1. W Herroelen & R Leus, 2005. "Identification and illumination of popular misconceptions about project scheduling and time buffering in a resource-constrained environment," Journal of the Operational Research Society, Palgrave Macmillan;The OR Society, vol. 56(1), pages 102-109, January.
    2. Joel Goh & Nicholas G. Hall, 2013. "Total Cost Control in Project Management via Satisficing," Management Science, INFORMS, vol. 59(6), pages 1354-1372, June.
    3. Wendi Tian & Erik Demeulemeester, 2014. "Railway scheduling reduces the expected project makespan over roadrunner scheduling in a multi-mode project scheduling environment," Annals of Operations Research, Springer, vol. 213(1), pages 271-291, February.
    4. Chen, Bo & Hall, Nicholas G., 2021. "Incentive schemes for resolving Parkinson’s Law in project management," European Journal of Operational Research, Elsevier, vol. 288(2), pages 666-681.
    5. Martens, Annelies & Vanhoucke, Mario, 2019. "The impact of applying effort to reduce activity variability on the project time and cost performance," European Journal of Operational Research, Elsevier, vol. 277(2), pages 442-453.
    6. Kim, Byung-Cheol, 2022. "Multi-factor dependence modelling with specified marginals and structured association in large-scale project risk assessment," European Journal of Operational Research, Elsevier, vol. 296(2), pages 679-695.
    7. R L Bregman, 2009. "Preemptive expediting to improve project due date performance," Journal of the Operational Research Society, Palgrave Macmillan;The OR Society, vol. 60(1), pages 120-129, January.
    8. Guofeng Ma & Jianyao Jia & Tiancheng Zhu & Shan Jiang, 2019. "A Critical Design Structure Method for Project Schedule Development under Rework Risks," Sustainability, MDPI, vol. 11(24), pages 1-20, December.

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