How to Use the Critical Path Method to Manage a Project

Project team brainstorming at a whiteboard

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Project managers use the term "critical path" to describe the entire sequence of tasks that must be done on time if a project is to be completed in the minimum amount of time required. Crucially, it includes the assumption that each one of the tasks must be completed before the next can begin, so any delay in any task will delay the entire project.

The critical path method, or CPM, is used to plan a project by defining each necessary step in it and estimating how long each will take. The goal is to prevent the kind of bottlenecks that can plague any complex plan.

Defining the Steps

Project managers work with team members to define all of the work required to complete a project or to achieve the project scope.

The work is typically broken down into units called work packages. These work packages are small enough to be associated with an owner, managed for risk, and controlled for time, cost, and materials. A common benchmark is that a work package should take no less than eight hours and no more than 80 hours to complete. 

Each work team defines and estimates the time and cost necessary to deliver their work packages.

Diagramming the Results

The project manager then pools the packages and sequences them in the order in which they must be completed.

The result is a network diagram that identifies key measures: 

  • Early start: The earliest a work package can start
  • Early finish: The earliest a work package can be completed
  • Late start: The latest a work package can be started and not delay the project
  • Late finish: The latest a work package can be finished and not delay the project
  • Slack or float: The amount of time a work package can be delayed and not impact the project

These metrics are used to calculate the various possible paths through this network of packages. Any slack time can be identified.

The project manager and team members adjust the various paths and look at different options until they are sure they have found the most efficient and timely and least risky project plan. 

  • It is important to note that the critical path can change based on resource scheduling, even during the execution of a project.

Why Use This Method

The critical path helps the project manager and team focus their efforts on the most important work packages.

It also serves as a reference tool for monitoring and reporting progress and adjusting resources as needed. Project managers use it even mid-project to identify work that can be fast-tracked to avoid delays in the overall project.

Really Complicated Projects

When a project is large and complex, the project manager may wind up with a network diagram that has multiple critical paths or one critical path and several near-critical paths. This is described as a "sensitive" project network. The more sensitive it is, the greater risk there is of delays.

Critical Path Creation Software

Small projects lend themselves to a manual calculation of the critical path.

The largest initiatives may contain literally thousands or tens of thousands of work packages. In those cases, project managers rely on project management software programs to calculate and describe the project network diagram and critical path or paths.

Nonetheless, it is important that the project manager understands how to develop, refine, and manage the critical path with or without such tools.