What is the critical path method?

The critical path method (CPM) is a project planning technique that helps determine scheduling flexibilities and identify the crucial tasks for project completion. In project management, the critical path is the longest sequence of tasks your team must finish to complete a project from A to Z. When you encounter delays in critical tasks, the remainder of your project will suffer delays. Deploying CPM helps you discover the most significant tasks in your timeline, identify task dependencies, and calculate task durations for enhanced team and project success.

Morgan Walker of DuPont partnered with James Kelley of Remington Rand to develop the Critical Path Method in the 1950s. They aimed to overcome costly plant shutdowns and restarts due to inefficient scheduling. The ingenious method of breaking down complex projects into a series of single tasks to better understand a project’s flexibility has become a popular planning process for project managers today.

Why use CPM?

The critical path method in project management provides valuable insight into planning projects, scheduling tasks, and allocating resources. Consider the following reasons for implementing CPM in your project management strategies.

Avoid bottlenecks

Bottlenecks are the bane of project managers, so avoiding them wherever possible is typically at the top of their list of priorities. Using a network diagram and plotting out dependencies through critical path analysis gives you a better idea of which tasks can run in unison and which can’t, so you can save time and schedule accordingly.

Save time on planning future projects

The critical path method improves future planning using experience from past projects. Comparing expectations with actual progress in current and past projects provides crucial data you can use for future project planning.

Better resource management

CPM offers project managers more insight into task priorities and timelines, allowing them to facilitate resources more accurately, such as personnel, tools, equipment, funding, and technology.

Finding the critical path

To find the critical path of your project, you must look at the critical and non-critical activities and their duration. There are six steps to accomplish this.

List activities

List all project tasks or activities leading to your end goal using a work breakdown structure, which serves as the basis for the CPM. In the following example, the project’s end goal is an online magazine article, with a breakdown chart listing tasks in order of logistics and duration of each task.

Identify dependencies

Using the breakdown structure as your foundation, determine which tasks depend on others. Determining your task dependencies helps you identify activities that can work in parallel with others. Here’s what task dependencies look like using the magazine example.

• Task B2 depends on A1.
• Task C3 depends on B2.
• You can do task C3 and D4 in parallel.
• Task E5 depends on D4.
• Task F6 depends on C3, D4, and E5.

You will use the dependent tasks as an activity sequence to determine your critical path.

Create a flowchart

Take your breakdown structure and turn it into a network diagram (a flowchart that displays the chronological order of activities) by putting each task into a circle or box and using arrows to show dependencies.

Estimate task times

You can’t determine your critical path without knowing the duration of each task. When in the planning stages of a project, you must estimate task times. You can do this in several different ways.

• Estimate times based on accumulated data from past projects.
• Use industry standards as a baseline for duration.
• Make your best-educated guess based on knowledge and experience.

The forward and backward pass technique is an alternate option.

• Forward pass: A technique that moves you forward through your flowchart to determine a project’s duration while finding the critical path of the project.
• Backward pass: A technique where you move backward from the end result to calculate late start (LS) or find any slack (float) in the activity.

Calculate the critical path

There are three steps for manually calculating the critical path of a project.

• Document the start and end times next to each task.
  • The first task has a start time of 0; the task’s duration is the end time.
  • The start time of the next task is the finish time of the previous task, which makes the end time the start time plus the duration of the current task.
  • Continue this for all tasks.
• Observe the end time of the last task in the sequence to calculate the duration of the entire sequence.
• The sequence of tasks with the longest duration will be your critical path.

Once you have your critical path, you can base your project schedule around it.

Calculating the project float

The term float, also called slack, is the amount of duration flexibility a given task has. Float determines the maximum activity delay before it impacts a subsequent activity or the project end date. Calculating the slack of each task gives you an idea of the total flexibility of the project. Project managers use float to cover unexpected issues and potential project risks. You can’t get float from critical tasks as they have set durations you shouldn’t change. In contrast, you can delay non-critical tasks as they won’t affect your project completion date.

Free float vs. total float

There are two types of float to consider.

• Free float is the length of delay a specific activity or task can have without impacting the following task. Free float only exists if two or more tasks share a common successor. For example, it’s where activities converge on a network diagram.
• Total float is the amount of delay an activity can have without violating a schedule constraint or delaying the project end date.

Understanding float is beneficial to project managers in a few different ways.

• Allows for prioritization: Identifying tasks with free float provides a better idea of which activities have more delay flexibility and which ones you should prioritize.
• Helps keep projects running on time: Keeping track of the total float helps determine if your project is running on track. The larger the float, the more chance you have of finishing your project on time or early.
• A usable resource: Having float means you can cover unexpected issues or potential risks. When you know precisely how much float you have, you can choose the best way to use it.

Using the critical path method

CPM provides your team full transparency into your project’s progress for enhanced task and completion time monitoring. There are times when you may require additional applications of CMP.

• Meeting alternate deadlines: Though not ideal, there are some cases when you must push up your project timeline. When this happens, you can use one of two schedule compression techniques: fast tracking (using your critical path to determine and run parallel processes to speed up the overall duration) or crashing (allocating additional resources to speed up activities). Plotting your critical path can help you choose the best strategy for meeting updated deadlines.
• Data compilation for future projects: CMPs consist of educated guesses for task durations. Comparing your original critical path to the actual critical path while the project runs allows you to learn and adjust accordingly. This valuable data helps estimate future task durations more accurately.

Gantt chart vs. the critical path method

While Gantt charts and CPM show dependencies between tasks, the two have some differences.

• Gantt chart: Using data displayed as a horizontal bar chart, the Gantt chart shows how project tasks progress and the required resources for each activity by plotting activities on a timescale.
• Critical path method: Displayed as a network diagram with linked boxes, CPM visualizes all critical and non-critical paths to calculate project duration. However, it doesn’t show any required resources and plots activities on a network diagram without a timescale.

Get the best of both worlds by pairing Gantt charts with CPM to track critical paths and keep your projects running on schedule.