Critical path method (CPM): steps, float, examples

 Editor’s Note: This is an educational article on using the critical path method in project management. For specific Asana functionality, please see our project management hub page.

Building out a project roadmap helps you visualize everything that needs to happen to reach your end goal. The critical path method does exactly that by mapping out the key tasks necessary to complete a project.

This technique allows you to manage task dependencies and set realistic timeframes. Read on to learn how the critical path method works and how you can use it with your team to optimize project timelines.

What is the critical path method (CPM)?

The critical path method (CPM) is a project scheduling technique that identifies the longest sequence of dependent tasks required to complete a project. Also called critical path analysis (CPA), it helps you determine which tasks must finish on time to avoid delaying the entire project.

CPM focuses on three core components:

• Critical tasks: The most important activities in your project timeline

• Task dependencies: The relationships between activities that determine sequencing

• Task durations: The estimated time required for each activity

CPM was developed in the late 1950s as a methodology to address increased costs resulting from inefficient scheduling. Since then, CPM has become popular for project planning and task prioritization. It helps you break down complex projects into individual tasks and gain a better understanding of the overall project flexibility.

Why use critical path analysis?

CPAs can provide valuable insight into how to plan projects, allocate resources, pace projects towards milestones, and schedule tasks.

Here are some reasons why you should use the critical path method:

• Improves future planning: CPM can be used to compare expectations with actual progress. Data from current projects can inform future project plans.

• Facilitates more effective resource management: CPM helps project managers prioritize tasks, enabling them to better avoid resource constraints.

• Helps avoid bottlenecks: Bottlenecks in projects can result in lost valuable time. Plotting out project dependencies using a network diagram will give you a better idea of which activities can and can't run in parallel, allowing you to schedule work accordingly.

Limitations of the critical path method

While the critical path method is a powerful tool, it's important to know its limitations. Understanding these helps you make better scheduling decisions and avoid potential pitfalls.

• It requires accurate estimates. The effectiveness of CPM depends on how accurately you can estimate the duration for each task. Inaccurate estimates can make the entire critical path unreliable.

• It can be rigid. For dynamic projects where priorities shift frequently, a fixed critical path can quickly become outdated. It works best for projects with a well-defined scope and sequence of activities.

• It doesn't account for resource availability. CPM focuses on time, not the people or equipment needed to complete tasks. You'll need to use separate techniques, like resource leveling, to manage resource constraints.

When to use the critical path method

The critical path method is most effective in specific situations. Consider using it when your project involves:

• Complex dependencies. If your project has many tasks that must be completed in a specific order, CPM helps you visualize and manage that sequence.

• Well-defined activities. CPM works best for projects with clearly defined, predictable tasks, such as construction, manufacturing, or engineering.

• Strict deadlines. When on-time completion is the top priority, CPM shows you exactly which tasks to monitor to prevent delays.

You'll typically apply CPM during the planning phase to establish a clear project schedule and set a baseline for tracking progress.

How to find a project's critical path

Finding the critical path involves identifying the longest path between the project's start and end by comparing the durations of critical and non-critical tasks. Below is a breakdown of the steps, with examples.

1. List activities

Use a work breakdown structure to list all the project activities or tasks required to produce the deliverables. The list of activities in the work breakdown structure serves as the foundation for the rest of the CPM.

For example, let's say the marketing team is producing a new interactive blog post. Here are some tasks that might be in the work breakdown structure:

Once you have a high-level idea of everything that needs to be done, you can start identifying task dependencies for the whole project.

2. Identify dependencies

Based on your work breakdown structure, identify the tasks that depend on one another. This will also help you identify any work that can be done in parallel with other tasks.

Here are the task dependencies based on the example above:

• Task B is dependent on A

• Task C is dependent on B

• Tasks C and D can run in parallel

• Task E is dependent on D

• Task F is dependent on C, D, and E

The list of dependent tasks is called an activity sequence and is used to determine the critical path.

3. Create a network diagram

The next step is to convert the work breakdown structure into a network diagram, a flowchart that displays the chronology of critical path activities. Create a box for each task and use arrows to depict task dependencies.

You'll add other time-bound components to the network diagram until you have the general project schedule figured out.

4. Estimate task duration

To calculate the critical path, the longest sequence of tasks, you first need to estimate the duration of each activity.

To estimate the duration, try:

• Making educated guesses based on experience and knowledge.

• Estimating based on previous project data.

• Estimating based on industry standards.

Alternatively, try using the forward pass and backward pass technique:

• Forward pass: Calculates the earliest start time (ES) and earliest finish time (EF) for each activity. Start with 0 at the first activity and work forward through the schedule using EF = ES + duration.

• Backward pass: Calculates the latest start (LS) and latest finish (LF) dates. Start with the last activity and work backward through the schedule using LS = LF - duration.

The early and late start and end dates can then be used to calculate each task's float, or scheduling flexibility.

5. Calculate the critical path

Calculating the critical path can be done manually, but you can save time by using a critical path algorithm instead.

Step 1: Write down the start and end time next to each sequence of activities to calculate the sequence's "duration."

• Find the start time of the first activity in the sequence.

• Find the end time of the last activity in the sequence.

• The duration is the end time of the last activity minus the start time of the first activity.

Step 2: Determine the number of dependencies along each sequence.

Step 3: The sequence of activities with the longest duration (end of sequence date - beginning of sequence date) is the critical path. If multiple sequences of activities have the same duration, the sequence with the most dependencies is the critical path.

Using the same example above, here's what the critical path diagram might look like:

Once you have the critical path identified, you can build the project schedule around it.

6. Calculate the float

Float, or slack, refers to the amount of flexibility of a given task. It indicates how much the task can be delayed without impacting subsequent tasks or the project end date. Float helps you gauge the flexibility of your project. You can use it to cover project risks or unexpected issues.

Here's what float values tell you:

• Zero float: Critical tasks with fixed dates that cannot be delayed.

• Positive float: Non-critical tasks that can be delayed without affecting the project completion date.

Calculating the float can be done with an algorithm or manually. Use the calculations below to determine the total float and free float.

Here's a breakdown of the two types of float:

• Total float: The amount of time an activity can be delayed from its early start date without delaying the project finish date or violating a schedule constraint. Total float = LS - ES or LF - EF.

• Free float: This refers to the time an activity can be delayed without affecting the following activity. Free float is possible only when two or more activities share a common successor. Free float = ES (next task) - EF (current task).

There are a few good reasons why project managers benefit from having a good understanding of float:

• It keeps projects on time: Monitoring a project's total float helps you determine whether it's on track. The bigger the float, the more likely you'll finish early or on time.

• It allows you to prioritize: By identifying activities with free float, you'll have a better idea of which tasks should be prioritized and which can be postponed.

It's a useful resource: Float is extra time that can be used to cover project risks or unexpected issues. Knowing how much float you have allows you to choose the most effective way to use it.

How to use the critical path method

Critical path methodology provides visibility into your project's progress, allowing you to monitor tasks and their completion times. Below are some additional applications of CPM.

Compress schedules

Though not ideal, there are times when project deadlines may be pushed up. In those situations, you can use two schedule compression techniques: fast tracking and crashing.

• Fast-tracking: Examine the critical path to identify activities that can be performed simultaneously. Running parallel processes will reduce overall execution time.

• Crashing: This process involves allocating more resources to speed up activities. Before obtaining more resources, make sure that they are still within the project scope and let the stakeholders know of any changes.

Having the critical path plotted out can help you choose the appropriate strategy to meet updated deadlines.

Resolve resource shortages

Keep in mind that CPM doesn't account for resource availability. When there is a resource shortage, like an overbooked team member or a lack of equipment, you can use resource leveling techniques to solve the issue.

These techniques aim to resolve resource overallocation issues and ensure a project can be completed with the resources currently available. Resource leveling adjusts project start and end dates, so you may need to reassign the critical path or apply it to activities with float.

Compile data for future use

The schedule created from CPM is subject to change since you're working with educated estimates for activity durations. You can compare the original critical path to the actual critical path as the project runs.

This data can serve as a reference to obtain more accurate task duration estimates for future projects.

Critical path method vs. PERT

CPM and Program Evaluation and Review Technique (PERT) were both developed in the 1950s. PERT is used to estimate uncertainty around project activities by applying a weighted average of optimistic and pessimistic factors.

PERT uses three time estimates to find a range for the duration of an activity:

• Most likely estimate (M)

• Optimistic (O)

• Pessimistic (P)

The calculation for PERT is: Estimated time = (O + 4M + P) / 6

The main difference between PERT and CPM is their level of certainty around activity durations. PERT estimates the time required to complete activities, whereas CPM is used when activity durations are already known.

Differences aside, both PERT and CPM analyze the following components:

• List of required tasks

• Estimated duration for each task

• Task dependencies

These two project management tools can be used in tandem to boost their effectiveness. You can use PERT to obtain more realistic estimates of task durations before calculating the critical path and floats.

Critical path method vs. Gantt chart

Gantt charts are horizontal bar charts that map project activities and can be tracked against a set timeline. Both CPM and Gantt charts show task dependencies.

Here's how the two tools compare:

Gantt charts can be paired with CPM to track critical paths over time and keep your project running on schedule.

Use your project's critical path for better project management

CPM can be a useful asset in project management, particularly for planning tasks and managing resource constraints. With the help of project planning tools, you'll be able to create schedules and track projects with ease. Ready to put the critical path method into action? Get started with Asana to visualize your project's critical path and keep your team aligned.

Frequently asked questions about the critical path method