Critical Path Method (CPM) in Project Management

The critical path method

The critical path method
The critical path method - CPM

Critical Path Method (CPM) in Project Management

The critical path method (CPM) is a fundamental concept in project planning and management. It is a technique used for scheduling project activities like PERT method. Critical path of a schedule demonstrates the activities that can not be delayed. Because any delay in the critical path causes the delay of the project. In other words, in order to complete the project on time, activities on the critical path must be completed on time. This article discusses the Critical Path Method by the help of an example.

History of the Critical Path Method



The critical path method was developed by James E. Kelley and Morgan R. Walker as a scheduling technique in the 1950’s. It was developed for the US Navy Polaris Missile Program like the PERT method. Although both techniques serve the same purpose, they use different methods to calculate the activity duration. Gantt charts which did not involve activity relationships were in use before the development of critical path and PERT methods. Since it was developed in the DuPont Company in 1957, the critical path method has become an efficient tool for tracking the project schedule.

The critical path method is widely used in todays projects. It is applicable to all kinds of projects from various industries.

Concept of the Critical Path Method

The concept of the critical path method is discussed widely in the PMBOK Guide. Therefore it is an important concept for the PMP Exam. According to The Project Management Body of Knowledge (PMBOK), The Critical Path is “the sequence of scheduled activities that determines the duration of the project.”

Briefly in this method, activities are listed, activity durations are determined, dependencies between the activities are established and the longest path is identified by making forward and backward calculations.



In a schedule network diagram, many paths can be seen starting from a point and ending at a point. These paths consist of activities which have durations and dependencies. Every path have some duration but the critical path is the longest path of the network diagram. In other words it is the shortest time possible to complete the project

Key Steps of the Critical Path Method

Below steps are used for implementing the critical path method.

1. List the Activities

Activities can be determined by using the work breakdown structure and project scope. The scope of work defines the deliverables required to complete the project. The work breakdown structure divides the project scope into meaningful work packages.

2. Establish Dependencies

Activity dependencies represent the relationship between the activities. Predecessor and Successor activities should be determined in this step. Below questions should be answered;
Which activity will start before this activity ends?
Which activity will finish at the same time as this activity ?
Which activity will start after this activity ends?

3. Draw the Network Diagram

After the identification of activities and establishing their dependencies, a network diagram can be drawn. Below figure illustrates a simple network diagram.

Critical Path Method-Network Diagram
Critical Path Method-Network Diagram

4. Estimate Activity Completion Time

Activity durations can be estimated by using past experiences, productivity analysis and three- point estimation method.

Unlike the Critical Path Method, The PERT Method relies on there point estimation. In three-point estimation, activity durations can be determined by making the estimations below;
– Most Likely Estimate: The most likely estimate
– Optimistic Estimate: The best-case estimate
– Pessimistic Estimate: The worst-case estimate

5. Identify the Critical Path




Forward and backward pass calculation is used to determine the critical path and total float. By the help of Forward Pass and Backward Pass calculations, the earliest start and finish dates, and the latest start and finish dates for each activity can be identified and the Critical Path of the network diagram can be determined.

Critical Path
Critical Path of the Network Diagram

Advantages of the Critical Path Method

The following are a few benefits of the critical path method:

  • It improves decision making within the project team
  • It is a visual technique which enables to show activities, activity dependencies and durations in the same diagram
  • It enables the project team to make time optimizations.
  • It enables to manage and organize large and complex projects
  • It enables to calculate The Earliest Start/Finish and The Latest Start/Finish dates in order to manage activities and procurement tasks.
  • It helps to determine which activities can be delayed without delaying the project
  • Schedule compression methods such as fast tracking and crashing rely on the critical path method.

Limitations of the Critical Path Method

Below are the limitations of the Critical Path Method



  • It is hard to manage activities in large and complex projects without softwares.
  • It does not consider resource allocations.
  • Activity durations should be determined correctly orherwise, the critical path of the project will be wrong.
  • It will be hard to determine the critical path if there are many other similar duration paths in the project.

Summary



The critical path method (CPM) is a strong tool in project time management. It is used widely for project planning and updating purposes by specialists. This method gives the project management teams correct completion dates for their projects. However it is not easy to apply this method to large scaled projects that have thousands of activities without support of a software. Softwares like Primavera P6 and Microsoft Project can bu used to determine the critical path of a project easily.

External References
[1] Kelley, James; Walker, Morgan. Critical-Path Planning and Scheduling. 1959 Proceedings of the Eastern Joint Computer Conference.

See Also

The Critical Chain Method

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