This paper examines the Critical Path Method (CPM), a project management tool originally developed by DuPont in the 1950s and now widely used in construction, architecture, and engineering. The paper explains how CPM models project activities as a network of nodes and arcs, how project managers use it to identify the critical path and calculate float times, and how the method supports cost-benefit analysis when weighing speed against expenditure. It also addresses the human dimension of large construction projects, including competing stakeholder interests among owners, general contractors, and subcontractors, and how CPM knowledge helps managers negotiate effective compromises.
The Critical Path Method (CPM) was originally developed by the DuPont Company in the 1950s as a project management tool used when DuPont shut down its chemical plants for maintenance work. The shutdown process was complex and detailed, and had to be deployed in a highly regimented fashion in terms of completing steps and coordinating the workforce (CPM, 2011, Net MBA). Yet the process also demanded a certain degree of flexibility for managing unexpected delays, as well as the need to reduce costs. Ever since its inception, companies have used the CPM method to optimize efficiency and reduce costs. Today it is most often applied in the construction, architectural, and engineering industries to manage projects with multiple components.
The method is graphical in nature and is therefore ideal for multi-stage projects. "CPM models the activities and events of a project as a network. Activities are depicted as nodes on the network, and events that signify the beginning or ending of activities are depicted as arcs or lines between the nodes" (CPM, 2011, Net MBA). At the beginning of the CPM process, project managers specify the individual activities required to ensure the project's completion, sequence those activities, and create a diagram with an estimated overall completion time. They then identify the "critical path" — the longest sequence of dependent activities from start to finish. The model is updated as the project progresses.
"The basic CPM terms are Early Start and Early Finish (collectively known as Early Dates), Late Start and Late Finish (collectively known as Late Dates), and Total Float (the amount of time that an activity can float between the Early and Late Dates without delaying the project completion date)" (About CPM, 2011, PMSB).
Ideally, there should be a relatively narrow range of time between late and early start dates. Projects in which various nodes have a wide range of flexibility for delay are more likely to be completed on time and are therefore less likely to incur additional costs. "The available Total Float can be used to efficiently allocate resources or to accommodate unforeseen events or scope changes. Total Float is generally considered a shared resource between team members. Once the Total Float is consumed, additional planning adjustments (relationships and/or duration estimates) must be made, or the project completion will be delayed" (About CPM, 2011, PMSB).
"Chart symbols, numbered nodes, and dependencies"
"Which tasks cause costly delays if late"
"Balancing speed, labor costs, and budget"
"Owners, contractors, and subcontractor interests"
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