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
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.
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 a 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.
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. Here's a rundown on how to calculate critical path in your 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:
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 technical 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