Schedule Development

Critical Path Method (CPM)

The Critical Path Method (CPM) is a way to determine the critical path of a schedule. The critical path is the longest path of scheduled activities that must be met to execute a project.  This is important for Program Managers (PM) to know since any problems on the critical path can prevent a project from moving forward and be delayed.  Earned Value Management (EVM) analysis focuses on the critical path and near-critical paths to identify cost and schedule risks. Other schedule paths might have slack time to avoid delaying the entire project, unlike the critical path. There might be multiple critical paths on a project. The method is used with Program Evaluation and Review Techniques (PERT).

Critical Path Method (CPM) Goal

The goal of the Critical Path Method (CPM) is to determine the critical path in order to focus resources more efficiently. The method helps Program Managers (PM):

  • Understand which tasks are on the critical path to determine a completion date
  • How best to manage tasks around the critical path to meet the scheduled end date
  • Focus on preventing schedule slips
  • Gain insight when planning work packages and tasks

Determining the Critical Path

The Critical Path is determined when analyzing a project’s schedule or network logic diagram and uses the Critical Path Method (CPM).  The CPM provides a graphical view of the project, predicts the time required for the project, and shows which activities are critical to maintaining the schedule.

Critical Path

Critical Path Method (CPM) Steps

The seven (7) steps in the CPM are: [1]

  • Step 1: List of all activities required to complete the project (see Work Breakdown Structure (WBS)),
  • Step 2: Determine the sequence of activities
  • Step 3: Draw a network diagram
  • Step 4: Determine the time that each activity will take to completion
  • Step 5: Determine the dependencies between the activities
  • Step 6: Determine the critical path
  • Step 7: Update the network diagram as the project progresses

The CPM calculates the longest path of planned activities to the end of the project and the earliest and latest that each activity can start and finish without making the project longer. This process determines which activities are “critical” (i.e., on the longest path) and which have “total float” (i.e., can be delayed without making the project longer). [1]

Difference Between Critical Chain Method (CCM) and Critical Path Method (CPM)

There are few differences between the Critical Chain Method (CCM) and the Critical Path Method (CPM) but they both focus on schedule development and estimation. These are:

  • Critical Path Method is focused on how long a project is based upon task estimation and the amount of float on a project.
  • Critical Chain Method operates on shortening how long a project is due to overestimation, uncertainties, and buffers.

Critical Path Method (CPM) and Program Evaluation and Review Technique (PERT)

Program Evaluation and Review Technique (PERT) is a method used to examine the tasks in a schedule and determine a Critical Path Method variation (CPM). It analyzes the time required to complete each task and its associated dependencies to determine the minimum time to complete a project. It estimates the shortest possible time each activity will take, the most likely length of time, and the longest time that might be taken if the activity takes longer than expected.

Although CPM and PERT are conceptually similar, some significant differences exist mostly due to the type of projects best suited for each technique. PERT is better to use when there is much uncertainty and when control over time outweighs control over costs. PERT handles uncertainty of the time required to complete an activity by developing three estimates and then computing an expected time using the beta distribution. CPM is better suited for well-defined projects and activities with little uncertainty, where accurate time and resource estimates can be made. The percentage of completion of the activity than can be determined. [1]

Critical Path Method History

The CPM scheduling technique was introduced at approximately the same time as PERT Analysis. It was developed by J. E. Kelly of Remington-Rand and M. R. Walker of DuPont to aid in scheduling maintenance shutdowns in chemical processing plants. Over the years, CPM has enjoyed more use than any other network scheduling technique. It is based on the concept of critical path and was designed to focus on the time and resources, particularly cost, necessary to complete a project’s activities.

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Updated: 6/30/2021

Rank: G9.3

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