non-Critical Path Schedule Delays
FEATURED PAPER
By Niall McShane
Washington State, USA
Abstract
An algorithm is presented which extends the critical path method with new metrics at the task and path level to identify and prioritize non-critical path delays and guide the project manager to intervene when such delays threaten the project completion date. This method does not rely on statistical simulation (e.g. Monte Carlo) techniques or require a-priori knowledge of potential risk and is responsive to early warning signs of delays within the project. It has particular applicability in projects where the scale and complexity of the project network precludes any individual understanding all of the detail in the project schedule.
Keywords: Monitoring & Control; Project Networks; Risk, Uncertainty
Introduction
Critical Path Method or CPM has been around for almost 70 years. Starting in late 1956, the Integrated Engineering Control Group at DuPont, with support from Remington Rand Corporation, began to research the possibility of using newly emerging computer processing capabilities to overcome challenges in traditional project planning and scheduling procedures for large scale construction projects. The approach used activity on arrow diagrams to map out dependencies between tasks and attached durations and costs to the activities to allow schedule and cost analysis to be performed. The longest path through the network, the Critical Path, could be calculated and the cost of alternative schedules, achieved by crashing task durations to improve schedule or allowing non-critical tasks to float to level resources, could easily be determined. The results of this work were published by Kelley and Walker (1959). Concurrently, the US Navy was working with Booz, Allen & Hamilton on a similar problem related to research and development projects associated with the Polaris Fleet Ballistic Missile program (Malcolm et al, 1959). Where duPont’s emphasis was on managing cost and schedule for deterministic schedules, the Navy’s concern was with the unpredictability that was inherent in research and development projects. Consequently, while the Navy’s Program Evaluation Review Technique or PERT shared a lot in common with CPM, including the use of activity on arrow diagrams and the concept of a critical path, PERT used three point estimates to compute an expected duration for each task and attempted to predict a probabilistic critical path but lacked a cost model or the idea of analyzing cost-schedule tradeoffs. Over time, these competing methods converged, adopting the best practices of each method, especially as software tools emerged to support project scheduling. This hybrid critical path method quickly spread to construction and other industries and became a de-facto standard in the project manager’s toolkit. However, as early as the 1960’s people were already recognizing that both CPM and PERT inadequately addressed the risk of schedule delays; in particular delays to non-critical path activities.
A key development in the early 1960’s was the introduction of Monte Carlo simulation (Van Slyke, 1963) which extended the schedule analysis to include a large number of simulations of the project network using task durations that are derived randomly from the range of possible durations estimated for each task. A key output of this type of schedule analysis was a criticality index which reflects the probability that a particular task will become critical during project execution. This helped to expose areas of the schedule which were non-critical in the deterministic view but which had high risk of becoming critical if some combination of tasks in those paths took longer than their expected duration.
Further advances in trying to manage the inherent risk of variability in schedule durations such as cruciality (Williams,1992) and schedule sensitivity (Elmaghraby, 2000) attempted to quantify not only the probability that certain tasks or paths could become critical, but also the anticipated impact that such developments would have on the overall project schedule. Another key development in the late 1960’s was the US Federal Government’s cost/schedule control systems criteria which evolved into a more flexible approach of earned value analysis, EVA or earned value management, EVM (Anbari, 2003) which is used to track variances in the cost and schedule performance of the project.
A more comprehensive critique of critical path based approaches was offered by Goldratt (1997) who argued that CPM is inherently flawed because it focuses on managing tasks instead of managing uncertainty, resources and human behavior. His solution proposes to cut task estimates to a minimum, replace total float and free float with project buffers and feeding buffers, eliminate multi-tasking and define a critical chain which considers both scheduling and resource constraints. Management of the project proceeds by managing resource contention and tracking the rate of buffer consumption. This approach requires significant cultural change within an adopting organization and there is some controversy over whether this is truly a revolution in project management or a management fad that is difficult to implement in practice (Herroelen et al, 2002 and Raz et al, 2003).
While these approaches have helped to mitigate risk, they all require a level of insight or skill to define the degree of uncertainty in a project’s tasks or a level of cultural change and trust between workers and management that many project based organizations cannot achieve. Even if these skill levels or cultural norms can be achieved, there may still be truly unexpected events which the planning team failed to recognize. As project size and complexity increase, it becomes increasingly difficult for these methods to adequately address the risk.
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How to cite this paper: McShane, N. (2026). An Algorithm for Identifying and Prioritizing non-Critical Path Schedule Delays, PM World Journal, Vol. XV, Issue III, March. Available online at https://pmworldjournal.com/wp-content/uploads/2026/03/pmwj162-Mar2026-McShane-Algorithm-for-identifying-and-Prioritizing-non-critical-path-delays.pdf
About the Author
Niall McShane
Washington State, USA
Niall McShane is a retired senior manager and corporate executive. He first managed projects at the European Space Agency in the 1980’s as an employee of Logica Space and Defence Systems Ltd before joining Motorola as a project manager, project management advocate and manager of project offices. He was responsible for several significant accomplishments in project management at Motorola including the establishment of an Engineering and Project Management Council which set standards for a career ladder in project management, and the creation of a project management framework for the corporation. Later in his career he served as an engineering manager with Motorola and as a test manager and customer success executive with IBM.
Niall lives near Seattle and enjoys sailing and playing Pétanque. He can be contacted at nialljmcshane@gmail.com
