critical path method

A guide to using the critical path method

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  • Agile and project planning

Everything you do is part of a process—a series of steps or tasks that get you a desired output or help you to reach a specific goal. Some processes are more rigid and require you to follow a series of steps in strict order. Other processes might be more relaxed, allowing you to complete tasks out of sequence to achieve the desired output.

The tricky part of any process is deciding where to prioritize each task. When prioritizing tasks, you’ll want to look at the dependencies, the tasks that must be completed before the next one can start.  

The critical path method can help you to identify and prioritize your most important tasks and dependencies. Knowing which tasks are most important and in which order they must be completed helps you to create a critical path for your project.

What is the critical path in project management?

The critical path method (CPM), or critical path analysis, is a technique for scheduling complex projects that was developed in the 1950s. The technique lets you identify the most important (or critical) activities and their dependencies in a project.

Project managers for all types of projects (software development, construction, manufacturing, engineering, etc.) use CPM to prioritize critical tasks, identify dependencies, and estimate the duration of each task. This lets them create more realistic schedules for completing the overall project.

The longest sequence of these critical tasks and their dependencies represents the critical path for the project. Other activities that are important to the project, but don’t have dependencies, can be completed out of sequence or in parallel with the critical path.

How does CPM differ from PERT?

The program evaluation and review technique (PERT) is another scheduling tool that was developed in the 1950s. Like CPM, PERT is used to identify the tasks needed to complete a project, but its focus is more on estimating the amount of time needed to complete each task, and the overall minimum time needed to complete the entire project.

PERT estimates the time it takes to complete each task by using the weighted average of the most optimistic estimate (O), the most pessimistic (P), and the most likely (M).

The main difference is that PERT uses uncertainty to estimate task durations while CPM is used after durations have already been estimated. So you might see PERT being used in conjunction with CPM.

critical path method

Read our guide to learn more about PERT and how it compares to other project management tools.

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Why is identifying the critical path so critical?

Any given project might have multiple processes comprised of multiple tasks. If you don’t identify the project’s critical path, you might spend too much time on side activities that could be completed at any time. This could cause delays that might jeopardize your project.

For example, let’s say you have the goal of getting to work on time. The process breakdown might be as simple as:

  1. Wake up early enough to get ready.
  2. Take a shower.
  3. Get dressed.
  4. Eat breakfast.
  5. Get to the train station.
  6. Get on the right train.
  7. Get off at the right stop.

Each task comes with its own process and set of tasks. For example, the getting dressed process might be: 

  1. Put on your shirt.
  2. Put on your pants.
  3. Put on your socks.
  4. Put on your shoes.

These are the most important tasks you need to do so you leave your house clothed. These tasks do not need to be followed in the order presented. The only dependency seems to be that the socks need to be on before the shoes. You could try to put on the socks after the shoes, but you probably won’t get the result you want. 

In this process, you have allotted a certain amount of time for getting dressed. Some days you might get dressed faster than other days. This means you might have some extra time (slack or float) to work on a parallel task, such as putting on a belt, before you need to move to the next task.  

There might be other tasks associated with getting dressed such as putting on a tie, putting on a hat, putting on a jacket, etc. These tasks can be completed during slack time or later in the process. For example, you might put on a tie as you ride the train. But if you spend too much time picking out a tie, you might have to skip breakfast and head to the train station.

So, identifying the critical path is important because it helps you to:

  • Prioritize tasks: These are the tasks that need the most attention to avoid any delays in your process.
  • Identify slack: This is the total amount of time an activity can be delayed without causing delays to the entire project.
  • Efficiently manage resources: Knowing the critical path and the tasks that run in parallel gives you a better idea of the slack available in every task. You can assign more experienced employees to the critical tasks to ensure the project stays on schedule. Employees who are still learning certain skills can be assigned to parallel tasks to give them more time to learn on the job.
  • Visualize dependencies: Create a diagram of your critical path and its parallel tasks. This lets you easily see where the dependencies are and what needs to be done when. This also gives you clarity into where resources need to be allocated to avoid costly idle time.
  • Plan for the future: While monitoring the progress of a project, you can compare expectations against the actual results. This information can help you as you plan future projects because you can more easily identify and fix potential bottlenecks and other problem areas that could cause delays.

How to find the critical path

You can find the critical path in any type of project using the following process.

Step 1: Identify all tasks and activities

Here is where you create a master list of all of the processes, tasks, and activities that need to be completed in your project. This work breakdown structure template can help you to break down a large project into smaller, more manageable processes and tasks. This helps everybody to understand dependencies and helps employees to understand how and where their work fits in the overall project.

critical path method
Work breakdown structure template example (click on image to modify online)

Step 2: Identify dependencies

From your list in the work breakdown structure, identify the tasks that can’t start until the previous task is completed. 

Step 3: Estimate time requirements

You need to estimate the time it will take to complete each task along the critical path. Your estimates can be based on experience with similar tasks in other projects, data from past projects, and so on. Consider using PERT to get a more accurate estimate by calculating the average of the most optimistic, pessimistic, and most likely time estimates.

Step 4: Create a critical path diagram

A critical path diagram makes it easy for your teams and stakeholders to visualize the project’s critical path. The diagram helps people to understand the workflow, the dependencies, tasks that can be done in parallel, what’s been done, and what needs to be done.

Step 5: Calculate the critical path 

On your critical path diagram, add the start and end times next to every task. The start time for the first task is 0. The end time for that first task represents the duration to complete that task. For example, if your first task is estimated to take 20 hours to complete, you would add the number 0 and the number 20 to that task. 

The start time for the next task is the end time of the previous task. The end time is the start time plus the duration of the task. For example, if the second task is estimated to be completed in 10 hours, you would add the number 20 (the end time of the previous task) and the number 30 (20 + 10) to that task.

After you have done this for each task, you will know the total estimated time for completing the entire critical path.

Step 6: Consider slack and resource allocations 

Slack, also called float, is the total amount of time a task in a sequence of dependencies can be delayed before it causes delays in the project. By determining where you have slack in a critical path, you can reassign team members to other tasks to keep the work flowing. For example, if a task was estimated to be completed in 20 hours, but it was completed in 10, you potentially have 10 hours of idle time for the resources assigned to that task. You can assign them to parallel tasks to keep the work moving. 

Or, you can use slack to determine how long you can wait before starting a task if the previous task was delayed for some reason.

critical path method

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