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 sequence of activities that have little or no room for delay in a project. Crucially, it includes the assumption that each one of the tasks in the path must be completed before the next can begin, so a 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 and managed for risk. They can then be controlled for time, cost, and materials. A common benchmark is that work packages 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 (ES): The earliest a work package can start
  • Early finish (EF): The earliest a work package can be completed
  • Late start (LS): The latest that a work package can be started and not delay the project
  • Late finish (LF): The latest that a work package can be finished and not delay the project
  • Slack or float: The amount of time a work package or activity 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, timely and least risky project plan. To use the following formulas, the values are substituted by the day the value occurs, such as the fifth day of the project.

Total Float is the amount a task can move without disrupting the project finish date.

LF - EF or LS - ES = Total Float

Free float is the amount a task can move and not disrupt any other tasks.

ES2 - EF1 = Free Float

Where ES2 is the early start of the task directly following the early finish of its previous task, EF1. So, if task five finishes early on day four, and task six starts on on day five, you have one day of free float (5 - 4 = 1).

The critical path of the project can be determined by choosing the tasks, and creating a task network diagram. This diagram shows all the tasks by order of completion. If you had five tasks, you could label them V, W, X, Y, and Z.

Task V is the initial task. Task Y needs W and V to be completed, but task X does not. Task Z needs Y and X to be completed, while X only needs V to proceed. Determine a duration for each of the tasks.

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

If task V early starts on day one and requires 3 days to complete, calculate your late start of V using:

LS = ES + Duration - 1

Then, you can use task V's LS to calculate the ES of your next task, W.

ES2 = LS1 + 1, or 1 + 3 - 1 = 3

You should complete this for every task you have, which give you the early start dates for your tasks. To calculate the early finish dates use this formula, starting with your last task:

EF = LF - Duration + 1

You then calculate the late finish of the previous task:

LF2 = EF1 - 1

Work your way backwards to your first task. Once you have completed that, you should calculate the total float for your tasks using the formula for the total float from above.

The tasks with zero float are on your critical path. This means a late finish for any tasks with zero float affects the project end date. Tasks not equal to zero can flex the number of days calculated by the total float and not affect the 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 can use this method mid-project to identify work that can be fast-tracked to avoid delays in the overall project.

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 can have the critical path manually calculated. Larger initiatives may contain literally thousands or tens of thousands of work packages.

In those cases, project managers generally rely on project management software to calculate and describe the project network diagram and critical path or paths.