What is the Critical Path Method, and How Do I Use It on Real Projects?

The critical path method is a way of creating and interacting with your project’s schedule. It works on waterfall projects (projects where tasks and timelines are planned in detail at the beginning of the project), or waterfall portions of hybrid projects (projects that are a mix of waterfall and agile approaches).

Are these elements true of your project?

1. Tasks can be defined at the beginning of the project.
2. Task durations can be estimated with reasonable accuracy.
3. The timeline or deadline of the project is very important.

If so, the critical path method will be a truly helpful method to understand and use.

But this method won’t make much sense unless you’re familiar with Gantt charts, so let’s start there.

What is a Gantt Chart?

A Gantt chart is a way of displaying a project’s tasks in sequence from left to right. Each task is represented by a horizontal bar with a length proportional to how long the task is expected to take. The real magic of a Gantt chart is its ability to show dependencies, or which tasks can’t start until one or more other tasks are complete. Each dependency is shown by an arrow going from the end of the task that needs to be finished (the predecessor task) to the beginning of the task that’s waiting to begin (the successor task). Successor tasks are placed immediately after their predecessor tasks on the timeline, so the overall project can be completed in the shortest possible time.

This is what a Gantt chart looks like:

 And this is a Gantt chart you might see on a project complex enough to need a project manager:

What is a Critical Path? 

The reason we needed to look at Gantt charts first is because the critical path method is all about paying attention to the critical path through your project’s Gantt chart.

The PMBOK® Guide – Seventh Edition defines a critical path as:

“The sequence of activities that represents the longest path through a project, which determines the shortest possible duration.”

Put another way, the critical path shows the minimum amount of total time a project needs based on the longest sequence of tasks that have to wait for each other to start, one after the other.

Here’s what the critical path looks like on our sample Gantt chart, highlighted in red:

The red “path through the Gantt chart” is the critical path because if any of these tasks starts late or takes longer than planned, all the other tasks on the critical path will be pushed later and cause the overall project to take longer than the 17 days estimated on this chart (when Task K is expected to be complete).

Can you see how that’s not the case with the tasks framed in black? Even though Task C is only estimated to take 4 days (from day 2 until day 6), the next task that’s waiting on it to begin—Task I—can’t start until day 12 anyway because it’s also waiting on other tasks. So if Task C took longer than estimated to complete—even up until day 12—the overall project wouldn’t be delayed. This flexibility for tasks not on the critical path to be delayed extra days without causing a delay in the overall project is called float—they can “float” to the left or right on the timeline up to a point without affecting anything else.

But past a certain point, if a task in black takes longer than its allotted float days and does start to delay the overall project, it becomes part of the critical path, and some tasks previously on the critical path might not be any longer.

For example, if Task I took 3 days (pictured below) instead of 1 day (pictured above), it would push Task K later, and now the whole project would take 18 days, not 17. This also adds Task G and Task I to the critical path, and it takes Task H off the critical path: even though a delay of Task H would push Task J later, it wouldn’t immediately affect the new completion estimate for the overall project:

What is the Critical Path Method?

Now that you have this context, let’s look at the PMBOK® Guide – Seventh Edition’s definition of the critical path method:

“A method used to estimate the minimum project duration and determine the amount of schedule flexibility on the logical network paths within the schedule model.”

Can you see how this works?

You can “estimate the minimum project duration” because you’ve put every task on a timeline with a start date as early as possible—each task immediately following its predecessors—and you can see what overall projected completion date this gives you.

You can “determine the amount of schedule flexibility” for any given task: if it’s on the critical path, it has no schedule flexibility unless you’re okay delaying the overall project. If it’s not on the critical path, delays are usually acceptable until it bumps into a task on the critical path—then you would have a problem.

Using the Critical Path Method on Real Projects

The critical path method is ultimately a way to do two things:

1. Prioritize your attention as a project manager

If I am supporting a project where the timeline is very important, I watch tasks on the critical path very closely and check in with the people doing those tasks very frequently (with an honest explanation of why I’m checking in frequently to help them not take it personally). I check on non-critical-path tasks less frequently.

With a Gantt chart that has five times or twenty times as many tasks as the sample above, shortcuts to good prioritization like this one really help a project manager cut through the chaos and focus on the actions most likely to help the project meet its goals.

2. Find opportunities to solve project problems with minimum schedule impact

What if a person or a resource needed for a task suddenly becomes unavailable? With the critical path method, you know that:

• You need a solution more urgently if the affected task is on the critical path. If a person is out sick for one day, and their task isn't on the critical path, you might not have any real problems.
• If you have limited people or resources, tasks on the critical path are often the best place to assign them. You can keep the critical path moving on schedule, and you’ve bought yourself some time to work on other solutions for non-critical-path tasks.

There are other methods to solving schedule problems, such as adding additional people or resources to a single task (crashing) or starting a successor task before a predecessor task is complete (fast-tracking). I'll save detailed explanations of these methods for another post, but understanding your project’s critical path is a critical (ha) first step to making a decision about when to use these or other schedule modification methods.

Whether you hoped to understand the critical path method for your PMP or another certification exam, or whether you are hoping to use it on the projects you're managing, I hope you now have both a technical and practical grasp of the idea. It is such a mainstay of the way I think about waterfall projects that I’m not sure what I’d do without it.