The Big Picture: Critical Path Method and PERT
To control large projects, operations managers need to closely monitor resources, costs, quality, and budgets. They also must be able to see the “big picture”—the interrelationships of the many different tasks necessary to complete the project. Finally, they must be able to revise scheduling and divert resources quickly if any tasks fall behind schedule. The critical path method (CPM) and the program evaluation and review technique (PERT) are related project management tools that were developed in the 1950s to help managers accomplish this.
In the critical path method (CPM), the manager identifies all of the activities required to complete the project, the relationships between these activities, and the order in which they need to be completed. Then, the manager develops a diagram that uses arrows to show how the tasks are dependent on each other. The longest path through these linked activities is called the critical path. If the tasks on the critical path are not completed on time, the entire project will fall behind schedule.
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To better understand how CPM works, look at (Figure), which shows a CPM diagram for constructing a house. All of the tasks required to finish the house and an estimated time for each have been identified. The arrows indicate the links between the various steps and their required sequence. As you can see, most of the jobs to be done can’t be started until the house’s foundation and frame are completed. It will take five days to finish the foundation and another seven days to erect the house frame. The activities linked by brown arrows form the critical path for this project. It tells us that the fastest possible time the house can be built is 38 days, the total time needed for all of the critical path tasks. The noncritical path jobs, those connected with black arrows, can be delayed a bit or done early. Short delays in installing appliances or roofing won’t delay construction of the house because these activities don’t lie on the critical path.
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Like CPM, the program evaluation and review technique (PERT) helps managers identify critical tasks and assess how delays in certain activities will affect operations or production. In both methods, managers use diagrams to see how operations and production will flow. PERT differs from CPM in one important respect. CPM assumes that the amount of time needed to finish a task is known with certainty; therefore, the CPM diagram shows only one number for the time needed to complete each activity. In contrast, PERT assigns three time estimates for each activity: an optimistic time for completion, the most probable time, and a pessimistic time. These estimates allow managers to anticipate delays and potential problems and schedule accordingly.
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