Critical path method (CPM)

The critical path method (CPM) is a project management technique for process planning. It defines the tasks as critical or noncritical to ensure steps are taken in a timely fashion. CPM is most useful for projects that have many tasks involving complex interaction. The critical path itself is the sequence of activities that must be completed on schedule for a project to meet its deadline. Activities are dependent on other activities being completed before the next step can begin. Some projects may have more than one critical path. CPM allows a project manager and others involved to quickly see the elements of a project that are critical to advancing to the next step. CPM was developed by the DuPont chemical company and Remington Rand and first used in 1958 on a chemical plant construction project.

Background

Project management has an ancient history. The ancient Egyptians had detailed and complicated plans for moving each enormous block used to build the pyramids. In modern times, project management may involve multiple departments and activities. Worksite preparation, materials procurement, and the presence of various types of workers are some of the factors that must be organized to keep a project on schedule and budget.

The DuPont chemical company and Remington Rand Univac, a computer firm, began developing CPM in 1956. They used the UNIVAC-1 computer to create a graphical representation of the project to predict the schedule of completing of each task and the project as a whole. They tested CPM in 1958 on a construction project and again the following year during a maintenance shutdown of a plant in Kentucky.

At the same time, the US Navy was working with aerospace company Lockheed Missile Systems and consultancy firm Booz Allen & Hamilton on a project management system for the Polaris missile program. They developed the Program Evaluation and Review Technique (PERT), which was calculated on an IBM computer. The Polaris project involved more than three thousand contractors, and PERT was designed to chart their activities related to the time needed to complete tasks. PERT did not address cost or quality issues, as CPM did for commercial projects.

PERT is used to track large-scale, complex projects. PERT charts or diagrams indicate each task as an arrow. Dates for achieving milestones are represented as circles and categorized with four time estimates: optimistic, pessimistic, likely, and expected. In their most basic forms and functions, both PERT and CPM are very similar. However, commercial projects often find CPM more useful because it emphasizes minimizing cost and includes an additional quality review of each stage before moving on to the next phase. PERT is commonly used in research and development, while CPM, which includes hard deadlines, is a standard in construction and industry. In many cases, project managers choose to use both systems.

Early CPM diagrams used arrows to represent tasks. These were connected by nodes indicating sequencing. This was known as the activity-on-arrow (AOA) method. During the 1960s, Stanford University professor John W. Fondahl developed a new format for representing the tasks in CPM. Fondahl proposed using nodes—squares, circles, or ovals—for the tasks. Nodes were connected by lines or arrows indicating the sequence. This activity-on-node (AON) approach quickly became the standard.

During the 1980s, project managers sought to improve CPM. The system was not designed to take into account activities such as labor issues that could threaten projects and deadlines. It also did not have flexibility in changing a critical path midway through a project. In 1997, Israeli physicist Eliyahu Goldratt introduced the critical chain idea. This method focuses on project resources, costs, and flexibility in keeping a project on schedule, and allows project managers to insert unexpected problems.

Overview

The CPM is a flow chart that emphasizes the critical path, or the steps that must be completed on time for the project to meet its deadline. The CPM uses the flow chart to calculate the time the project needs based on estimates for each task in the sequence. This allows the team to understand the expected schedule and the project manager to track performance toward progress goals. Team members and managers can also see tasks that must be progressing simultaneously. If a deadline is missed, the team can quickly calculate how the delay affects project completion and look at individual tasks to see if any parts of the project can be completed more quickly to make up the lost time.

Developing the CPM involves six steps. First is identifying all the higher-level tasks and breaking them down into subtasks or sections. The CPM chart can be whatever style works for those involved, such as lists, tables, or tree charts. The second step is developing the activity sequence, which indicates dependencies. These are the tasks that rely on others being completed first. The correct order must be determined and clearly laid out. To do so, each task much be considered and organized according to the tasks that should be completed first, which tasks should be finished at the same time, and what should happen after this task. The third step is drawing the network diagram or critical path analysis (CPA). This may be done on paper or using special software. The CPA clearly shows the flow of tasks and all dependencies.

Step four is estimating the completion time of activities. This is usually based on similar projects—for example, in a construction project, the time needed to dig and pour a foundation of similar size can be estimated—or is calculated by someone with experience in such tasks. Time may be measures in days or weeks, depending on the task. If a person is unsure of estimates, they may use the three-point estimation method. This entails developing three time estimates for each task.

The fifth step is identifying the critical path or paths. This involves identifying the longest sequence of tasks in terms of time, the duration of days to complete tasks. Network sensitivity is likely to occur when a network has more than one critical path. This means that the chances of the critical path changing during the project increases. The sixth and final step is updating the CPM diagram as the project progresses. This includes using actual rather than estimated completion times and possibly identifying a new critical path. As tasks are completed, the project completion deadline may also be updated.

Bibliography

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