The critical path method (CPM) breaks down complex projects into task lists to determine which ones are critical to keeping a project on schedule. It was developed in the 1950s and is used to define a series of events that must be completed in the correct order and on schedule. CPM can be done manually or with computer software. It requires assumptions and constraints, including estimated duration, resources, and cost estimates. Commercial CPM software can show critical path changes in real-time, providing accurate results by regularly entering data for a project.
The critical path method (CPM) is a way to break down complex projects into task lists and determine which ones are critical to keeping a project on schedule. Developed in the 1950s, this method was used for complicated government and private industry programs that were running late for unspecified reasons. A critical path defined by the method as a series of events, called activities, that must be completed in the correct order and on schedule.
Construction or other projects may contain thousands of separate tasks that occur before a job is finished. Drawings have to be made, a construction site has to be prepared, and many other events have to occur even before construction begins. During construction, the foundation and structure of the building must be completed before interior work can begin. Some activities can run simultaneously, but some must wait for previous activities to complete. All these assumptions are used in the critical path method.
Critical path time calculations can be done manually, but can be quite difficult due to the many interactions between tasks. An easier way to create a CPM is to use a computer spreadsheet program. Since the late 20th century, there has been commercial software that will create critical path method results from manually entered task information.
Developing a CPM report requires the use of assumptions, because a project typically hasn’t started when the report is created. A project manager or programmer begins the process by listing all known activities from start to finish. Tasks are assigned constraints, if any, which can be the earliest calendar date a task can start or the latest date it can finish. These date constraints prevent tasks from moving through the model, and tasks that are constrained are often part of the critical path.
Along with any fixed calendar constraints, all tasks must be assigned an estimated duration or the days or weeks needed to finish. A task can have a long duration, such as building structural steel or refinishing the interior, which can affect the critical path. The software programs available for critical path method modeling will show when an activity has extended beyond an estimated time. Task durations can also affect the critical path if they take longer than the estimated project time allows. These results are summarized in the software report once all data has been entered.
Another function of critical path method software is to determine how people are used. Each activity can be assigned a resource, a person or a group. A task normally includes the name of the resource and the percentage of time required for the task. Reports can show when a resource is needed more than is available; for example, employees who are needed 150% of their allotted time. These constraints allow project managers to shift resources as needed to prevent under- or over-utilization of staff during a project.
Complex projects may have cost estimates included in the CPM report. Tasks can be listed with an estimated cost to complete and any additional cost if the task is late or extends beyond its due date. Changes to the critical path that lead to project delays or extensions will have a visible effect on the total cost of the project.
Commercial CPM software is often useful because it can easily show critical path changes for business data in real time. When a task changes in date or duration, a project manager can input the changes into the project schedule and immediately see the effects on the total project and critical path schedules, resources, and costs. A critical path method will provide the most accurate results by regularly entering data for a project as it continues and changing the task dates and durations to actual dates and completion times.
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