What Is the Critical Path of a Project?
In project management, the critical path is the longest sequence of tasks that must be completed to complete a project. The tasks on the critical path are called critical activities because if they’re delayed, the whole project completion will be delayed.
Finding the critical path is very important for project managers because it allows them to:
- Accurately estimate the total project duration
- Identify task dependencies, resource constraints and project risks
- Prioritize tasks and create realistic project schedules
To find the critical path, project managers use the critical path method (CPM) algorithm to define the least amount of time necessary to complete each task with the least amount of slack.
Once done by hand, nowadays the critical path can be calculated automatically with project scheduling software equipped with Gantt charts, which makes the whole CPM method much easier.
ProjectManager can calculate the critical path for you on our award-winning Gantt charts—learn more.
Now that we know what’s the critical path of a project, we can learn about the critical path method (CPM), an important project management technique that’s based on this concept.
What Is the Critical Path Method (CPM)?
The critical path method (CPM) is a technique that’s used by project managers to create a project schedule and estimate the total duration of a project.
The CPM method, also known as critical path analysis (CPA), consists in using a network diagram to visually represent the sequences of tasks needed to complete a project. Once these task sequences or paths are defined, their duration is calculated to identify the critical path, which determines the total duration of the project.
CPM History
The critical path method was developed in the late 1950s by Morgan R. Walker and James E. Kelley. The origins of the critical path method are closely related with the Program Evaluation and Review Technique (PERT), a similar method which is commonly used in conjunction with CPM.
Why Is CPM Important in Project Management?
Projects are made up of tasks that have to adhere to a schedule in order to meet a timeline. It sounds simple, but without mapping the work, your project scope can quickly get out of hand and you’ll find your project off track.
Using the critical path method is important when managing a project because it identifies all the tasks needed to complete the project, then determines the tasks that must be done on time, those that can be delayed if needed and how much float or slack you have.
When done properly, critical path analysis can help you:
- Identify task dependencies, resource constraints and project risks
- Accurately estimate the duration of each task
- Prioritize tasks based on their float or slack time, which helps with project scheduling and resource allocation
- Identify critical tasks that have no slack and make sure those are completed on time
- Monitor your project progress and measure schedule variance
- Use schedule compression techniques like crash duration or fast tracking
CPM Key Elements
Before we learn the steps to calculate the critical path, we’ll need to understand some key CPM concepts.
- Earliest start time (ES): This is simply the earliest time that a task can be started in your project. You cannot determine this without first knowing if there are any task dependencies
- Latest start time (LS): This is the very last minute in which you can start a task before it threatens to delay your project schedule
- Earliest finish time (EF): The earliest an activity can be completed, based on its duration and its earliest start time
- Latest finish time (LF): The latest an activity can be completed, based on its duration and its latest start time
- Float: Also known as slack, float is a term that describes how long you can delay a task before it impacts its task sequence and the project schedule. The tasks on the critical path have zero float, because they can’t be delayed
Let’s take a look at some critical path examples to better understand these critical path analysis elements.
Critical Path Examples
Here’s an example of a CPM diagram. Although it’s high-level, it can help you visualize the meaning of a critical path for a project schedule. For now, we’ll use this critical path diagram to explain the elements that make up the CPM method.
As you can see in this critical path diagram, project activities are represented by letters and the critical path is highlighted in green. Tasks F, G and H are non-critical activities with float or slack. We can also identify task dependencies between the critical path activities, and also between activities (A, F and G) or (A, H and E), which are parallel tasks.
Here’s another critical path example from Harvard Business Review, which shows a critical path schedule for the construction of a house. Each circle in the CPM diagram represents a project activity, as well as it’s duration, while the bolded arrows link the critical path activities. As projects become more complex, you’ll find more parallel tasks, like in this example.
How to Find the Critical Path of a Project in 8 Steps
Now that you know the key concepts of the critical path method, here’s how to calculate the critical path in 8 steps.
1.Collect Project Activities
Use a work breakdown structure to collect all the project activities that lead to the final deliverable.
2. Identify Task Dependencies
Figure out which tasks are dependent on other tasks before they can begin. Use your judgement and your team members’ feedback. Failing to define task dependencies correctly makes the critical path method useless.
3. Create a Critical Path Diagram
A critical path analysis chart, or network diagram, depicts the order of activities.
4. Estimate Timeline
To use the critical path method, you’ll need to estimate the duration of each task. Use data from past projects and other sources of information such as subject matter experts.
5. Use the Critical Path Algorithm
The critical path algorithm has two parts; a forward pass and a backwards pass.
Forward Pass
Use the network diagram and the estimated duration of each activity to determine their earliest start (ES) and earliest finish (EF). The ES of an activity is equal to the EF of its predecessor, and its EF is determined by the formula EF = ES + t (t is the activity duration). The EF of the last activity identifies the expected time required to complete the entire project.
Backward Pass
Begins by assigning the last activity’s earliest finish as its latest finish. Then the formula to find the LS is LS = LF – t (t is the activity duration). For the previous activities, the LF is the smallest of the start times for the activity that immediately follows.
6. Identify the Float or Slack of Each Activity
Use this formula to determine the float or slack of each task. Float = LS – ES
7. Identify the Critical Path
The activities with 0 float make up the critical path. All of these critical path activities are dependent tasks except for the first task in your CPM schedule. All project tasks with positive slack are parallel tasks to the critical path activities.
8. Revise During Execution
Continue to update the critical path network diagram as you go through the execution phase.
These critical path analysis steps determine what tasks are critical and which can float, meaning they can be delayed without negatively impacting the project schedule. Now you have the information you need to plan the critical path schedule more accurately and have more of a guarantee you’ll meet your project deadline.
You also need to consider other changes or constraints that might change the project schedule. The more you can account for these unexpected events or risks, the more accurate your critical path schedule will be. If time is added to the project because of these constraints, that is called a critical path drag, which is how much longer a project will take because of the task and constraint.
CPM Training Video
Still have questions about the critical path method? In this video, Jennifer Bridges PMP, explains how to find the critical path using a network diagram.
CPM & Project Management Software
As stated above, the critical path method (CPM) was first invented in the late 1950s. During those times, project scheduling software didn’t exist, and project managers had to calculate the critical path manually.
Fortunately, in our times there are many project management software alternatives that can help with the critical path process. Most of them use Gantt charts to represent CPM diagrams and calculate the critical path, but their feature sets vary greatly. One of the most commonly used project management software to identify the critical path is Microsoft Project. However, it has major drawbacks that make ProjectManager a better choice.
Here are some of the main features that you’ll need as a project manager to properly use the critical path method for your scheduling process.