What Is Critical Path Method in Project Management? CPM Examples Included

Monday, August 8, 2022
Michael Scheiner
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Welcome to this in-depth deep-dive look at critical path project management. If you’ve been meaning to learn all there is to know about the critical path method, you’re in the right place.

We’ll explain the critical path model, the history of CPM, and what it means specifically for project management as well as when it is most appropriate to use. 

Then we’ll get a bit nerdy and discuss the CPM algorithms for calculating time estimates and priorities, followed by a step-by-step guide on finding the critical path in your project planning. 

Sounds good? Then let’s get critical down that path!

 

The critical path method explained

For this explanation, we’ll keep it short, and then we’ll later unpack many of the key CPM terms and concepts. The CPM method is a project planning tool mainly used for scheduling and prioritizing tasks and work activities. 

CPM is distinguished from critical chain project management (CCPM) in that the latter focuses on buffers and resource management, as opposed to tasks. This also distinguishes it from the Six Sigma methodology, which focuses on streamlining specific aspects of the project (not the whole).  

To define critical path method further, it uses algorithms and other ways to separate critical tasks from non-critical tasks, to show the relationship between task dependencies, and most importantly, to determine the maximum completion time for the whole project.

These features of CPM let you clearly communicate the total estimated time for the whole project duration to your clients and other stakeholders. Being able to separate project management components increases efficiency. Owing to this pragmatic usefulness, CPM is one of the most popular project management approaches out there. 

Who created critical path analysis?

Critical path analysis has its beginnings in the horrid war times of the 1940s. It was the infamous DuPont company that applied some of the core concepts of CPM during work on the Manhattan Project.

Only in the 1950s did people begin to use the actual name “critical path method” or “critical path analysis.” This was due to the work of Morgan Walker at DuPont and his buddy James Kelley who worked over at Remington Rand. 

It was around the same time that the PERT method was being developed by the U.S. Navy, and the two systems do have things in common, and more on that below.

By the 1960s, CPM was used all around the world, but most importantly, in major construction projects. Most impressively, CPM was applied in the construction of the original World Trade Center Twin Towers in NYC. It’s also been widely used in other engineering industries, software development and plant maintenance.

Today, CPM must tough it out in a highly competitive arena of various project management methodologies. Though some of its contemporary practitioners may stray from some of its core tenets, there are still a lot of useful concepts that CPM brings to any complex project planning.

 

What is a critical path in project management?

Now let us dig deeper into the critical path definition. What is critical path method in relation to PM? 

The first thing you should know when discussing CPM is to know that it is a system for determining the project schedule from start date to end date using the longest duration for all tasks. This means that the timeframe you get using CPM should form the absolute maximum amount of time to complete the entire sequence of tasks.

The next part of a good CPM definition is to explain critical path activities in distinction from non-critical tasks.

You see, since CPM focuses on the most critical project activities as they are sequentially ordered for the entire project, it makes room for non-critical tasks to be able to be undertaken by team members with less of a strict placement on the project timeline. This concept is known as float, or slack. 

That is, non-critical tasks can float around the project timeline, as long as they are completed by project end date.

Then there is the idea of total float, which tells you how much time a task can be delayed before it affects the project end date, whereas regular or free float only talks about how much a predecessor task can be delayed before it affects the next task in the sequence.

Since CPM determines the longest sequence, it encourages project teams to apply various methods if the project completion date is fast approaching and the project scope is not nearly completed. 

For example, fast tracking is a method by which you can green light the parallel activity of doing several tasks at the same time that might have otherwise been done one at a time. Crashing is another method of saving time by shifting more resources, like labor-power or money, to certain work activities to try and get them done sooner. 

PERT and critical path method 

Similar to CPM is a critical path PERT chart. PERT stands for Program Evaluation Review Technique. Like CPM, it also helps estimate timeframes for your project’s sequence of activities. 

But whereas PERT uses an algorithm to arrive at the expected duration of a task with 3 different estimates, CPM templates focus mainly on the longest task durations. 

What’s more, PERT is predictive, and CPM is prescriptive, meaning CPM dictates how long the project team should be spending on work activities. Finally, both PERT charts and CPM charts are network diagrams with wireframes, but PERT uses milestones as nodes and tasks as connecting arrows, while CPM puts tasks as nodes.

If you want to do a deep-dive on PERT analysis, check out our article on the subject. You can also learn more about PERT vs CPM in our handy comparison guide.

Gantt charts and critical path method 

The famous Gantt chart often gets compared to a CPM chart. First of all, both charts look very different. As we’ve said, CPM is a flow chart or a network diagram showing the relationship between tasks as nodes. 

A Gantt chart is a bar graph chart showing the activity duration of tasks across a horizontal timeline. Gantt charts are great for seeing the sequence of tasks and which tasks will run parallel to others. 

Gantt templates are also useful for assigning tasks to team members and including other notes like priorities or resources.

CPM charts have the strong advantage of making dependent tasks easier to organize and communicate to the project team. CPM will use dates much less than Gantt charts, and instead focuses on milestones such as producing deliverables. 

When is critical path project management appropriate?

If you want to know more about when to use a project critical path as part of your planning and scheduling process, this section will list a few of the most appropriate cases. 

Streamline planning

As should be clear by now, critical path management is a powerful scheduling and planning tool. It excels at determining the longest possible overall project timeline so that there is less uncertainty about deliverables and milestones. 

Stakeholders appreciate this a lot. CPM is also useful for knowing your critical tasks, and which tasks have more float, or flexibility when it comes to their start dates. 

Improve resource management

Resource allocation is a central element of any project planning, perhaps up there right next to scheduling and time management. 

Between sorting out task priorities, parallel work activity and task dependencies, CPM makes it easy for managers to know how to allocate resources and when to shift resources if the situation calls for it.  

Avoid obstacles

Even though a critical path system is prescriptive, planning out the order and priority of tasks that team members will be following, all projects are subject to obstacles such as delays, bottlenecks, misallocated resources and more. CPM, with its use of fast tracking and crashing, can ease up the pressure from bottlenecks and other issues.

Project data

Because CPM is a project planning tool that involves a lot of work at the project start to set up the critical path chart, it is very beneficial for generating data in comparing actual project progress and results to the original project plan. This data of course is highly leverageable when making the next round of project plans and CPM charts.

 

How a critical path algorithm priority list works

Time to get into a bit of the old formula thinking, and CPM has some basic formulaic algorithms which are good to know about. Here are some key terms for this section:

  • ES: Earliest start time

  • LS: Latest start time

  • EF: Earliest finish time

  • LF: Latest finish time

One of the first algorithms associated with CPM is the one to determine the level of float in your project. The formula is:

Float = LS – ES

Meaning the amount of float, as expressed in a unit of time (hours, days, etc.) is the latest start time minus the earliest start time. That is, if a task can start as late as 40 hours into the project, and as early as 25 hours into the project, then this task can float for 15 hours after the earliest time without delaying the next task.

Forward pass and backward pass

The concepts of forward pass and backward pass are also algorithms used to determine things like slack, or for knowing how much forward or backward you can push a task.

The forward pass is used for figuring out the earliest start date by using the earliest finishing date of a predecessor task. The ES will always have as its highest value the EF of its predecessor, or in the formula:

EF = ES + time

That is, the earliest finishing time is the earliest start time plus the duration of the task. If task A is supposed to begin first thing Monday and has an earliest finishing date at the end of Tuesday, then task B’s earliest start time is first thing Wednesday.

The backward pass is the opposite, and it is used for the latest start date and the latest finish date of a task or whole project. Unlike the forward pass algo which goes from beginning to end, the backward pass algo works backwards from the finish time. The formula is: 

LS = LF - time

That is, the latest start time or date of a task is the latest finish time or date minus the duration of the task. For example, if a task will take 4 hours, and the latest it can be finished without delaying the project is noon, then the latest it can be started and still come in on time is 8 AM.

Now, let’s move on to a critical path calculation example to see how one finds the critical path in your project planning.

 

How to find critical path in project management

This section will discuss how to determine critical path. Remember, the word “path” here is key, as it is the path which makes the longest sequence of activities along the most critical tasks from the project start date to the project completion date. In other words, it is the longest path of all the critical activity duration. Here is how to calculate critical path:

  1. Scope and tasks

  2. Dependencies

  3. Create the network diagram

  4. Estimate durations

  5. Calculate the critical path

  6. Calculate the float or slack

1. Scope and tasks

In the first step toward determining the critical path duration, you need to have your project scope completed, and based on that, you must begin mapping out all the project tasks. Doing a work breakdown structure, or WBS, helps with this stage, as it will silo the tasks into parallel categories, and create groups for subtasks.

2. Dependencies

Once you have all the tasks mapped out, you must relate them in terms of their task dependencies. Some tasks must be finished before others can begin, while others cannot be completed until others have started. There can also be constraints here, like ASAP, ALAP and ‘must start by,’ and ‘must end by’ dates.

3. Create the network diagram

With all your tasks, dependencies and milestones, you can create your wireframe network diagram. Here, the nodes are the tasks and work activities, while the arrows show their sequential order and their dependencies. 

4. Estimate durations

After you've started your network diagram with tasks and milestones, it’s time to do the task time estimates and plug those figures onto the arrows. These estimates can be educated guesses, based on past project data, or using forward pass and backward pass formulas.

5. Calculate the critical path

To calculate the critical path, you count from the earliest start time of the very first task until the latest finish time of the final task. Or you look at the most amount of time from start to finish when considering the critical tasks of the project.

6. Calculate the float or slack

If you have your critical path mapped out, you can then determine the amount of free float for each task, which is ES - EF. You can also calculate the total float, which is LS - ES, or, LF - EF.

 

Project management critical path example

Let’s get into a critical path method example. For this CPM example, let's be very modern and stylish and think about “baking a cake to serve to friends.” Here your critical tasks are:

  1. Choosing their recipe

  2. Buying the ingredients

  3. Baking the cake

These tasks must be done in order and they are dependent on one another. Since we know the critical path in a project network refers to dependent tasks moving to project completion, this 1, 2, 3 sequence ticks all the boxes for being critical. 

Consequently, there are some non-critical tasks, like:

  • Inviting friends

  • Decorating the dining hall

  • Choosing a music playlist

These tasks can generally be done any time before the final stage of the project which is serving the cake to friends. 

Now, if it takes 1 hour to choose a recipe, 4 hours to go shopping for the ingredients, and 30 minutes to bake the cake, you can say the total duration of the critical path is 5.5 hours, or, if the earliest start time for you is 09:00 AM, then the estimated end time of the project according to the critical path is 02:30 PM.

Critical path diagram example

Critical path diagram example

Above is a CPM diagram example charting task dependencies in our cake-eating tale. Here the critical path in a project network is easy to see. As project complexity increases, finding the CPM can, accordingly, take more cognitive labor. 

 

Critical path schedule creation using tools

Right now, you might be asking if there is any project management software that helps with the critical path scheduling algorithm. 

And yes, there is. Vendors like Microsoft, Wrike, Asana and Monday.com offer some forms of CPM templates and scheduling functionality that lets managers plan and monitor project progress in real time. You can take a look at our project management software comparison table for some ideas on what’s out there and what’s good. 

 

Our conclusion on CPM project management

And there you have it, a winding trip down the critical paths of CPM. We’ve covered the ins-and-outs of the critical path model, the history of CPM, critical path examples, and its relationship to project management at large. 

You should now know how to use the CPM process and identify critical path in your project. 

Project scheduling activity is not always the easiest part of any project plan. However, with the critical path method, a bit of work at the outset goes a long way toward staying on schedule in the long run. 

Of course, as noted, implementing CPM will most certainly be taking place in the context of digital work tools. It’s a great idea to always consider CPM in relation to the apps you’re already using, or the ones you’re considering investing in. 

 

FAQs

In which stage in the project life cycle would critical path calculations be performed?

When implementing the critical path method in project scheduling, you calculate the critical path at the 5th stage of the planning process. This means you do this after you’ve estimated your task durations and created your network diagram, but before you calculate the float and slack.

What is CPM in project management?

In project management, CPM stands for the critical path method. CPM is a tool used for project planning and scheduling, particularly for calculating how long the entire project will take based on its most critical work activities. This is useful for knowing what tasks can float inside the schedule. 

Why is the critical path critical?

The critical path methodology is known for identifying critical tasks and work activities as opposed to non-critical tasks and work activities. The critical tasks form a sequence from start to finish, and this is the critical path which determines the maximum time one will spend on a project.

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