Critical Path Method (CPM) & PERT: Steps, Benefits, Techniques & Limitations

Critical path method or Critical path analysis

IN 1957, DuPont developed a project management method designed to address the challenge of shutting down chemical plants for maintenance and then restarting the plants once the maintenance had been completed. Given the complexity of the process, they developed the Critical Path Method (CPM) for managing such projects. It is a step-by step technique for process planning that defines critical and non-critical tasks with the goal of preventing time-frame problems and process bottlenecks. The CPM is ideally suited to projects consisting of numerous activities that interact in a complex manner. It helps you to plan all tasks that must be completed as part of a project. They act as the basis both for preparation of a schedule, and of resource planning. During management of a project, they allow you to monitor achievement of project goals. They help you to see where remedial action needs to be taken to get a project back on course.

 

Critical Path Analysis formally identifies tasks which must be completed on time for the whole project to be completed on time. It also identifies which tasks can be delayed if resource needs to be reallocated to catch up on missed or overrunning tasks. CPM is commonly used with all forms of projects, including construction, aerospace and Defense, software development, research projects, product development, engineering, and plant maintenance, among others. Any project with interdependent activities can apply this method of mathematical analysis. Although the original CPM program and approach is no longer used, the term is generally applied to any approach used to analyze a project network logic diagram.

The essential technique for using CPM is to construct a model of the project that includes the following:

1. A list of all activities required to complete the project (typically categorized within a work breakdown structure),
2. The time (duration) that each activity will take to completion, and
3. The dependencies between the activities.

CPM provides the following benefits 

1. Provides a graphical view of the project.
2. Predicts the time required to complete the project.
3. Shows which activities are critical to maintaining the schedule and which are not.

Steps in CPM Project Planning

1. Specify the individual activities.
2. Determine the sequence of those activities.
3. Draw a network diagram.
4. Estimate the completion time for each activity.
5. Identify the critical path (longest path through the network)
6. Update the CPM diagram as the project progresses.

CPM can help you to figure out

1. How long your complex project will take to complete
2. Which activities are ‘critical’ meanings that they have to be done on time or else the whole project will take longer.
3. If you put in information about the cost of each activity and how much it cost to speed up each activity
4. Its help to figure out whether you should try to speed up the project and if so what is the least costly way to speed up project.

CPM Limitations 

CPM was developed for complex but fairly routine projects with minimal uncertainty in the project completion times. For less routine projects there is more uncertainty in the completion times, and this uncertainty limits the usefulness of the deterministic CPM model. An alternative to CPM is the PERT project planning model, which allows a range of durations to be specified for each activity.

Program Evaluation Review Technique (PERT)

The Program (or Project) Evaluation and Review Technique, commonly abbreviated PERT, PERT is a method to analyze the involved tasks in completing a given project, especially the time needed to complete each task, and identifying the minimum time needed to complete the total project. It is commonly used in conjunction with the critical path method or CPM. A PERT chart is a project management tool used to schedule, organize, and coordinate tasks within a project. It was developed primarily to simplify the planning and scheduling of large and complex projects. 

It was developed for the U.S. Navy Special Projects Office in 1957 to support the U.S. Navy's Polaris nuclear submarine project. It was able to incorporate uncertainty by making it possible to schedule a project while not knowing precisely the details and durations of all the activities. It is more of an event-oriented technique rather than start- and completion-oriented, and is used more in projects where time, rather than cost, is the major factor. It is applied to very large-scale, one-time, complex, non-routine infrastructure and Research and Development projects. This project model was the first of its kind, a revival for scientific management, founded by Frederick Taylor (Taylorism) and later refined by Henry Ford (Fordism). DuPont Corporation’s critical path method was invented at roughly the same time as PERT.

Advantage of PERT

1. PERT chart explicitly defines and makes visible dependencies between the WBS elements.
2. PERT facilitates identification of the critical path and makes this visible.
3. PERT facilitates identification of early start, late start, and stock for each activity.
4. It provides for potentially reduced project duration due to better understanding of dependencies leading to improved overlapping of activities and tasks where feasible.
5. The large amount of project data can be organized & presented in diagram for use in decision making.

Disadvantages of PERT

1. These can be potentially hundreds, thousands of activities and individual dependency relationships.
2. The network chart to be large and unwisely requiring several pages to print and requiring special size paper.
3. The lack of a time frame on most PERT/CPM charts makes it harder to show status although colours can help (e.g. specific color for completed notes)
4. When the PERT/CPM charts become unwieldy they are no longer used to manage the project.

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Sandeep Ghatuary

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