How the Critical Path method is key in project management

What is the Critical Path Method?

The CPM tool is a powerful tool that can provide managers the ability to better control each project to a granular level, mapping out each individual task alongside their respective timeframes to give a more holistic overview of the project as a whole. Through this, managers will also be able to better manage budget allocation, scheduling, and even monitor progress more efficiently to ensure everything is on track. 

The Critical Path Method also helps managers analyze completed projects that used the tool to better see which tasks may have held up the process and how to better prepare for similar projects in the future, often known as “bottlenecks”. At times, you may hear other users note this as Critical Path Analysis, or CPA. Both tasks are essentially interchangeable and focus on the mapping out of key tasks that are necessary for a project’s completion. 

In an article for Harvard Business Review, writers F.K. Levy, G.L. Thompson, and J.D. Wiest describe the types of projects that are best for the use of the critical path method:

• Projects are ideally consisting of a well-defined set of tasks, activities, or jobs that have a clearly identifiable and measureable start and end points. 
• Each task, activity, or job has the capability to be started or stopped independently of each other (even those with dependencies), which separates itself from “continuous-flow processes” that are well-defined in their strict adherence to a specific routine.
• The jobs have a defined “sequential path”, or a pre-determined sequence for a particular overarching project to be completed. 

If your project fits these three criteria, you may be able to utilize the critical path method to better map out the internal tasks needed to fulfill it. 

Where did the Critical Path Method Start?

It was James Kelley and Morgan Walker of Remington Rand and DuPont, respectively, that gave rise to the first iteration of the Critical Path Method in the first place. Troubled with the issues of different production inefficiencies caused by shutdowns and other system problems, they needed to find a way to properly identify the production routes that remained “critical” to production so that they could adjust bottlenecks and delivery times accordingly. 

They both published the theory of their operation in their paper “Critical-Path Planning and Scheduling” back in 1959, at which point CPM began being used for other industry systems as well.

Key Concepts to Understand

Before you get to using the critical path method in your project, it’s important to introduce some foundational concepts that can help you better understand the different items that go into this analysis. 

Start and End Times

One of the most integral things to understand when it comes to the usage of the critical path methodology is to understand the different types of start and end times that come into the analysis of each task. Generally speaking, the start and end times of any tasks are intuitive enough in that the start time designates when something is started along the duration of a project, while the end times denote when that something ends. If illustrated into a formula, you can express the relationship between start and end times as:

Start Time + Duration of Task = End Time

Within the CPM analysis, you also have Earliest Starts and Latest Finishes. We will go into more detail about these two concepts below, but essentially the Earliest Start is how early a particular project can begin and the Latest Finish is the absolute longest a project task will take to be completed. 

Task Dependencies

As you may have experienced in your own project management, some tasks can only be started once you’ve completed a preceding one. In the critical path method system, you call this “task dependencies”, where a specific task needs to be denoted as following specifically from a previous task in order to begin. For example, you can’t cook an omelette without first whisking the eggs. 

Task dependencies are important not just in getting the total timeframe of a particular path within your project, but to also identify which tasks you can run in parrallel, or at the same time as another task. Doing this alone can help you uncover efficiencies you might not have realized before.

Float or Slack Time

Most projects you’ve likely worked on have a certain dimension of dynamism within each task. You may envision completing a certain leg of a project within one time frame, but end up finishing it in a longer time. Sometimes this causes a problem for your organization, so skillful project managers include what is colloquially known as “buffer” time to account for variables that might be out of their control. 

In the critical path method, this buffer time is known as “float” or “slack” time, which is the amount of time in a given project that can be delayed before it impacts the actual deadline requirement of a given project. 

Steps to Using the Critical Path Method

Diving deep now into the actual critical path method, you’ll need to follow roughly 6 different steps to properly execute a workable and robust critical path framework in your project management cycle. To better illustrate the critical path method’s different steps, we will use a hypothetical “Project A” as our example project for CPM analysis. 

Define Your Activities

Now pretend Project A is a project that has been done before by your company and that you are now tasked with managing this project on your own. You’re given information on the previous iterations of the project so you have a pretty good handle on the tasks required to complete it. 

Applying the critical path method to Project A will then need to start with the activities proper. In this first step, you’ll need to define the different tasks or jobs within Project A that are required for it to be completed. For our example, let’s imagine the following.

Following a structure defined as “Task - Duration (Hours)”:

• A - 3
• B - 5
• C - 5
• D - 7
• E - 5
• F - 3

As you can see, it’s important here to note down the expected duration it takes to complete a specific task. The duration timeframe itself can be expressed in minutes, hours, or even days, depending on the context of your project. For this Project A, we will use hours as our main period of time. 

Identify Any Dependencies

After you’ve laid out all the different activities within a project, you’ll need to start laying out the different dependencies that each task has. Remember, dependencies are tasks that need to be completed in order to move on to the next task in the chain. 

For Project A, let’s say that the dependencies are as follows:

• A - 3 - None
• B - 5 - A
• C - 5 - B
• D - 7 - A 
• E - 5 - D
• F - 3 - E,C

Based on the data above, we can see that Task A is our starting task as it has no dependencies, while tasks B and D follow branching lines from task A and essentially diverge the project path into two. Each path follows through with tasks C and E, respectively, until they eventually come together for final task F. 

Visualize Through a Network

You might have found it difficult to image the different steps presented above happening in sequential order, and that’s understandable as this is a shortcoming of explaining complex project tasks via exposition. That’s why a key component of the critical path method is to visualize the tasks in an organized manner. Most project managers utilize the Gantt chart for this process, but for our illustrative purposes, we will use a simple flowchart to visualize our Project A out. 

• A > B > C > F
• A > D > E > F

Now you can better see that there are two paths in our Project A, one that moves from task A to tasks B, C, and F, and another one that moves from task A to tasks D, E, and F. 

Plot Timelines Across Network

Now that we have a network of paths laid out, we need to input the timelines for each path. For ease of keeping track, you can lay out each task as a 2 x 3 grid, with the top left signifying the “Earliest Start”, the middle top showing the task, the top right showing the “Earliest End”, the bottom left showing the “Latest Start” and the bottom right showing the “Latest End”. Note that the starting task (Task A) will have 0 as both its earliest and latest start as its the first task in our sequence. 

Let’s start off with the first process in laying out these timelines, which is to use the “Forward Pass” method. With the forward pass method, you add up the start times with the durations to get the “Earliest End”, which you then repeat with each succeeding step. For example, our first path (Tasks A, B, C, and F) has a total duration of 13 hours, while our second path (Tasks A, D, E, and F) has a total duration of 15 hours. Note that reaching Task F will begin converging the two paths, in this case the starting time you will pick will be the one with the greater end time (you can’t start task F without finishing Task E, which takes a longer time to complete). 

You then work on developing your “Backward Pass”, which uses the latest start time of your last activity (in our case, Task F), and work backwards based off the end times indicated. For example, working backwards from our first path (Tasks A, B, C and F) will start off with 15 hours as the latest end for Task C, working all the way back to task B with a latest start of 5 by subtracting each duration to the end times. Working backwards from our second path (Tasks A, D, E, and F) will look very similar to the original first pass method, which you will understand once we begin analyzing the critical path. 

Identify the Critical Path

Now the critical path analysis is the goal of the critical path method, and is best characterized by by the longest duration path in your project, which will also have zero float or slack between its early and late start and end times. 

In our Project A, you may have noticed that the second path (Tasks A, D, E and F) have the longest duration of 15 hours. From this, we can understand that this path is our critical path, or the path that properly dictates the actual time needed to complete the project. It doesn’t matter if our first path (Tasks A, B, C, and F) are completed in an earlier schedule as the critical second path will still need to be completed to fulfill Project A’s requirements. You can double check this by using the Backward Pass method and identifying no float or slack times between starting and end times either on early or late start time frames.

Through the identification of the critical path, you can begin allocating resources to better manage this bottleneck in your process as this path remains a key signifier of how long your entire project will take. 

Determine Any Floats/Slacks

One more analysis you can do is to identify any floats or slacks within the process itself. Since we have identified our second path as our critical path, we also understand that this will take the longest to complete in our project. That, in turn, means that path one will have a little leeway in terms of float time to be completed as our Project A hinges on the completion of the critical path. 

In our example, the total float, meaning the amount of float time allowed for your team to add to the duration of a given path, is calculated by the Total Late Start - Total Early Start or Total Late Finish - Total Early Finish. In this case, our path one has a total float time of 2 hours, meaning that the task path can have an additional 2 hours of working time to complete. 

Key Takeaways from the Critical Path Method

The Critical Path Method is an incredibly powerful tool in defining the amount of time that is required to complete a project as well as to identify the different bottlenecks that are present in a project’s task list. The method above is a fairly manual method of calculating the different integral variables that help your analyzis, and can be expedited much faster with the usage of appropriate software that can calculate the different times in your critical path method. Remember, the utilization of online whiteboards can help you better map out processes like this for easier analysis.