What exactly is the critical path method in project management?
Critical path method (CPM) is a resource-utilization algorithm for scheduling a set of project activities. The essential technique for using CPM is to construct a model of the project that includes the following:
- A list of all tasks required to complete the project
- The dependencies between the tasks
- The estimate of time (duration) that each activity will take to complete
With this information, you can determine the critical path by identifying the longest stretch of dependent activities and measuring them from start to finish.
Once you’ve identified which activities are on the longest, or critical path, you can more easily discern which have total float, or can be delayed without making the project longer.
Using the Critical Path Method in a Project
Now we’ll try to demonstrate the concept of the critical path method with a simple, real-life example: planning a killer party. How should you plan and execute on this project?
1. Define the project scope
First, we need to define all of the tasks that must be finished to complete the project. For our party example, it might look like this:
- Choose a date and venue
- Make the ultimate playlist
- Set up the sound system
- Invite your friends
- Buy the food and drinks
- Cook your famous casserole
- Host the party
When we look at these tasks individually, we realize that some of them cannot be started before the others are completed. That is, some tasks are dependent on others. We’ve designated these relationships in the table below:
|Task Name||Dependent on|
|Choose a date and venue||-|
|Make the ultimate playlist||-|
|Set up your sound system||-|
|Invite your friends||Choose the date and venue|
|Buy the food and drinks||Invite your friends|
|Cook your famous casserole||Buy the food and drinks|
|Host the party||Casserole & sound system|
The actions “invite your friends,” “buy the food and drinks,” “cook your casserole,” and “host the party” form a sequence of tasks that must be performed in a specific order, one right after the other, to ensure a successful result. Such tasks are called sequential activities.
These tasks, together with the start of our project (“choose a date and venue”) are the most critical steps in completing our project. Thus, these actions will be placed on the critical path.
2. Critical path analysis and identification
The essential concept behind critical path analysis is that you can’t start certain tasks until others are finished. These tasks need to be completed in a sequence, with each stage being completed before the next stage can begin.
We’ve used Wrike’s Gantt Chart to show what a sample schedule can look like:
The critical path consists of the longest sequence of activities from project start to finish that must be completed to ensure the project is finished by a certain time. The activities on the critical path must be very closely managed. If jobs on the critical path slip, take immediate action to get the project back on schedule. Otherwise, the whole project can be delayed.
Imagine that you have a project that will take 30 days to complete. If the first activity on the critical path is 1 day late, the project will take 31 days to complete, unless another activity on the critical path can be completed 1 day earlier. The critical path essentially determines the end date in your project schedule.
3. Different project paths
You can have more than one critical path in a project, so that several paths run concurrently. This can be the result of multiple dependencies between tasks, or separate sequences that run for the same duration.
The critical path in project management may contain all the important activities associated with a project, or it may not. In fact, the activities on the critical path are not always the most important parts of the project. At the same time, there will be tasks that are not on the critical path, but that still determine your project’s success.
Understanding the critical path method involves determining which activities are critical to complete on time. But other activities that lie outside of the critical path may also be very important and require additional attention.
What Are Resource Constraints and Why Do They Matter?
Traditional critical path schedules in project management are based only on causal dependencies. We’ve already marked these dependencies in our plan. (e.g., it’s impossible to cook the casserole without buying the ingredients). However, a project may have limited resources that need to be taken into consideration. These limitations will create more dependencies, often referred to as resource constraints.
If you work on a team, you may split the project work between team members. In our example, while you’re choosing a date and venue and inviting people, one of your friends can make a playlist, and another can get the food and drinks. The tasks can be done in parallel, as on our chart above.
However, if you’re the only person responsible for the project, you have a resource constraint because you can’t be in two places at the same time. In this case, your critical path will look different.
On the chart above, we assume that you first need to choose the date and venue, and only later can you make a playlist. However, depending on the project conditions, these tasks can be performed in a different order.
This kind of critical path is called a resource critical path. This method was proposed as an extension to the traditional critical path analysis to allow for the inclusion of resources related to each activity. A resource-leveled schedule may include delays due to resource bottlenecks (i.e., unavailability of a resource at the required time), and it may cause a previously shorter critical path to lengthen.
Calculating the Length of Your Project
In project management, a critical path is the sequence of dependent tasks that form the longest duration, allowing you to determine the most efficient timeline possible to complete a project.
Getting back to our party example, let’s assume that you have to do everything by yourself. We estimated the length of time each activity will take. Also, we determined the approximate start time for each task on the critical path. Here’s what we came up with:
|Choose a date and venue||2 hours||Monday|
|Make the ultimate playlist||3 hours||Monday|
|Set up your sound system||1 hour||Monday|
|Invite your friends||2 days||Monday|
|Buy the food and drinks||1 day||Tuesday|
|Cook your famous casserole||2 hours||Wednesday|
|Host the party||2 hours||Wednesday|
Now if we add up all of our critical tasks’ duration, we’ll get the approximate time that we need to complete the whole project. In our case, 3 days and 6 hours, since “make the ultimate playlist” and “set up the sound system” are not on the critical path. If we add the duration to the start time, we can calculate the earliest project completion time. Understanding the CPM allows us to make this calculation quickly and accurately.
Flexibility in the Critical Path Method
The critical path method was developed for complex, but fairly predictable, projects. However, in real life, we rarely get to manage such projects. A schedule generated using critical path method techniques is often not followed precisely. As we already mentioned, any delay of an activity on the critical path directly impacts the completion date. New requirements may pop up, and new resource constraints may emerge.
Let’s say you’re planning to redecorate your living room with a friend. Your task list may look like this:
- Get rid of the old furniture
- Paint the walls
- Fix the ceiling
- Install the new furniture
Your friend’s responsibilities are to:
- Choose the new curtains
- Hang the new curtains
Parallel Tasks in the Critical Path
In our example, the curtain tasks form a sub-project and can be treated as a non-critical path. Your friend can “choose the new curtains” and “hang the new curtains” any time before the end of your project. The curtain tasks have flexibility in the start and end date, what is considered “float.” These tasks are parallel and will not be placed on the critical path. Here’s how this project would look on a Gantt chart:
If any of the parallel tasks were to be significantly delayed, it would prevent our whole project from being completed on time. Therefore, you should always keep an eye on parallel tasks.
Changes in the Critical Path
Now, let’s assume that choosing the curtains took our friend longer than we initially expected. This will delay the end of the project.
Our redecoration is incomplete without the new curtains, so the path that previously was non-critical becomes critical. The initial critical path changes.
To keep an eye on your non-critical tasks, keep your project schedule up-to-date. That’s the only way you’ll know exactly where your project is at any given moment and whether it will be delivered as initially planned.
The video below provides a great introduction to the critical path method (CPM).
Do you find the critical path method effective? Why or why not? Please let us know in the comments below.
Here are some further resources you can use to brush up on your critical path method knowledge:
- The Critical Path Method, Seminar, Stanford
- Explaining the Critical Path Method, U.S. Department of Health and Human Services
- The ABCs of the Critical Path Method by Harvard Business Review
- How to Use a Single Gantt Chart for Multiple Projects
- When to Use a Project Calendar vs. a Gantt Chart