September 28, 2022

Critical path in project management explained

Gantt Project Management
Jerzy Żurawiecki Content Specialist @BigPicture

Making sure your project gets done on time can be difficult, but some tools can help with that. Waterfall practitioners have been using the Critical Path in project management for decades for that very reason. Now, it’s time to learn about it and apply it to your initiatives.

What is the Critical Path in project management? How to determine it? What benefits does it offer to managers? These are the questions this article will answer. So read on and learn everything about the Critical Path.

Critical Path – the introduction

Simply put, the Critical Path is the longest sequence of tasks to be completed for the project to be delivered on time. In other words: which tasks are necessary to complete the project in a given time frame?

It stems from the Critical Path Method (CPM), a modeling technique that has been in use in project management since the 1950s. This approach is used primarily in Classic endeavors.

What is the goal of the Critical Path in project management?

This tool aims to identify the specific tasks that need to finish on time so the team can deliver the project without any delay.

Additionally, the Critical Path provides managers with another set of information; a list of tasks that can be delayed without compromising the project’s deadline.

What are the benefits of using the Critical Path?

Waterfall projects tend to last a long time, which poses a set of challenges for managers. Potential delays are just one of them. Estimating which tasks are critical in advance is invaluable in any project, regardless of duration.

Applying the Critical Path in project management awards managers of Classic projects a few perks:

  • provides a visualization of the project,
  • helps define which elements are critical to the success of the project,
  • supports the prioritization of tasks,
  • highlights the necessity to schedule a project at each stage,
  • helps manage available resources.

How to calculate the Critical Path in project management?

Just like a dish or a cake, the Critical Path has its recipe to make it work. Calculating the Critical Path is simple, but the actions involved require comprehensive preparation. However, they cover activities that managers perform before the project starts anyway, so it won’t burden you with much extra work.

  1. Specify each activity – list all the tasks of the project.
  2. Sequence the activities – list all dependencies between the tasks.
  3. Estimate the duration of each activity.
  4. Draw a Network Diagram – visualize the activities and their dependencies.
  5. Calculate Early Start and Early Finish using Forward Pass.
  6. Calculate Late Start and Late Finish for each activity using Backward Pass.
  7. Find the activities with matching Early Finish and Late Finish.
  8. Map out a sequence of these activities from step 7.

The steps themselves might not tell you much at first. But don’t worry; I’m going to break it all down with the help of an example.

Step 1: Specifying the activities

You must have done that countless times in your career as a project manager. After all, Classic initiatives require listing all tasks before the project launches.

Managers use a tool to visualize the scope of the project – the Work Breakdown Structure. It can serve as your point of reference in creating your list.

Our example has five activities:

  1. Activity A,
  2. Activity B,
  3. Activity C,
  4. Activity D,
  5. Activity E.

Naturally, most projects are much more complex. However, this short amount of activities will suffice to understand the Critical Path in project management.

Step 2: Mapping the dependencies

Oftentimes, one activity is dependent on the completion of another. Managers use visual boards to see the connections between tasks instead of simply writing them down. However, at this stage, we just need a list of connections. The visual aspect comes later. I’ve listed the dependencies of my sample project below.

A list of activities and their dependencies in an example project

Step 3: Estimating the duration of each activity

It’s difficult to predict exactly how long each task will take, but if you don’t try, you won’t even have a baseline. To keep all the essential information handy, it’s best to write down the activity list, dependencies, and durations in one place. You can use the table below for inspiration.

A list of tasks, dependencies, and durations for the example project

Step 4: Drawing a Network Diagram

The table above has all the required data, but seeing the whole picture is not that easy, is it? That’s why you need a visual tool. A simple diagram will do just fine.

The network diagram for our example project. All the dependencies are marked using lines

The dependencies are a bit clearer, aren’t they? Now it’s time to explain a few terms: Early Start, Early Finish, Late Start, and Late Finish. These metrics are indispensable in figuring out the Critical Path.

Step 5: Calculating Early Start, Early Finish with Forward Pass

The forward pass is a technique to determine the early start (ES) and early finish (EF) of each activity, as well as the duration of the entire project. It involves adding every activity’s early starts and finishes across the whole project. Here’s how it works in practice.

Activity A has an early start (ES) of 1. After all, it’s the start of the project. It is scheduled for two days, so it will finish at the end of day two (EF=2).

Activity B starts on day 3 and lasts for 5 days, so until the end of day 7. That means ES=3 and EF=7.

The next activity – C – starts at the same time as B (day 3) and takes eight days to complete. It should end on day 10. ES=3, EF=10.

The beginning of Activity D falls on day 8 since it’s dependent on Activity B and takes two days. ES=8, EF=9.

Then, we see a complication. Activity E has two dependencies – C and D. Meaning it can only start when both are complete. The earliest Activity E can start is when Activity C is over – the one that ends later. In this case, the later one ends on day 10, so Activity E starts on day 11 and lasts until the end of day 16. ES=11, EF=16.

I’ve updated the diagram with the ES and EF values to make it more digestible.

The network diagram with Early Start and Early Finish values

Now we know two things:

  • The duration of the entire project – 16 days.
  • The Early Start and Early Finish of each activity.

One of these activities has a bit of leeway. Activity D could finish a day later, and it wouldn’t delay the beginning of Activity E. This leeway is called Float or Slack.

Step 6: Calculating Late Start and Late Finish with Backward Pass

This technique considers how late an activity can start and finish without jeopardizing the final deadline and the dependent activities. It involves going backward – from the end to the beginning – hence the name.

We know that the end of the project is in 16 days, and not a day later, so that’s our Late Finish for activity E (LF=16). It takes 6 days to complete, so the Late Start is 11.

For Activity D, the latest possible finish is on day 10, so LF=10. As a two-day endeavor, it can’t start later than day 9, which means LS=9.

Activity C has to end before E because they are connected, so the Late Finish is day 10 (LF=10). It’s scheduled for 8 days, so it cannot start later than day 3 (LF=3).

Activity B can finish as late as day 8 and begin as late as day 4. Ergo, LF=8 and LS=4. Finally, Activity A must finish on day 2 to avoid blocking the following dependent activities.

You didn’t think I would forget about another diagram, did you? Here it is, with updated Late Start and Late Finish values:

The network diagram with Late Start and Late Finish values

What’s the takeaway from the backward pass? It helped us determine the Late Start and Late Finish of all of the parts of the project. Also, it allowed us to spot and calculate the Float.

Steps 7 and 8: Determining the Critical Path

Now, for the moment you’ve been waiting for. To determine the Critical Path, find the tasks with the same Early and Late Finish numbers. The sequence of these tasks is the Critical Path.

In the case of our example, the Critical Path consists of activities A, C, and E. See the image below:

The network diagram with the Critical Path marked in red

If you want to see the process broken down in a video format, check out the recording by Aileen Ellis, a project management certification trainer:

Critical Path in Project Management – is there a better way?

You might think: that sounds like a lot of work that I don’t have the time or energy to do. And why should you deal with that? PPM tools like BigPicture determine the Critical Path automatically. That way, you know exactly which activities in the project are crucial to its success and cannot be delayed. This knowledge can help you manage resources more precisely and prioritize surrounding tasks.

The PPM tools use Gantt chart software to visualize the Critical Path. As long as you have your scope and dependencies in the BigPicture app, you can generate the Critical Path for a Classic project with a single click. No need to do calculations on your own.

A screenshot of BigPicture with a Critical Path tasks marked in red
The red tasks on the Gantt chart make up the Critical Path.

Create alternative Critical Paths with scenarios

Interestingly, BigPicture takes this functionality a step further. It combines the Critical Path with what-if scenarios. You can think of them as time simulations – what a project would look like if you changed the dates of the activities. If the new dates impact the Critical Path, it will be updated automatically based on the latest data.

The scenarios allow you to visualize the timeline based on the changes made to a project. Adding the Critical Path to your scenario provides you with another data set to make informed decisions. For instance, if you want to create an alternative path for your project and see how a delay would impact its progress.

You can store scenarios as reference points and switch between live view and the scenario in seconds. It’s helpful in presentations to quickly compare versions during meetings and plan actions with an alternative in place.

The scenarios don’t make any changes in your Jira issues until you decide to replace your live view with it. Then, all the start and dates of a scenario will transfer into the original path of your project’s Gantt chart.

A screenshot of a what-if scenario in BigPicture
An example of a scenario in BigPicture. Compare it with the previous image. See any differences?

Now you are ready to determine the Critical Path for your projects. Of course, with the right PPM tool, you won’t have to do any math yourself, but at least you’ll know how this method works.