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Critical path method guide: How to use CPM [+ examples]


The critical path method (CPM) has been a project management mainstay since the 1950s that has gained a reputation for complex charts, symbols, and lots and lots of arrows:

image.png

Despite what this graph may imply, CPM is pretty intuitive. It helps the project manager answer a simple question:
Out of all the tasks in the project, which series of tasks takes the longest?
That series of tasks determines how long the project will take. Therefore, you should focus on completing those tasks on time so you can complete the whole project on time.

Armed with this knowledge, project managers can complete their project planning duties with a clear hierarchy of task prioritization, an estimate for how long the project will take to complete, and what, if anything, has the potential to throw the project schedule off. Let's dig deeper.

What is the critical path method (CPM)?

The critical path method is a project management strategy that helps identify task sequence to determine the duration of a project. When you use CPM, you list every task (i.e., project activities) involved with a project along with its predecessors, dependent tasks, and an estimate for how long it'll take to complete. Then, you use all this data to identify the project's critical path, which is the series of important tasks (called critical tasks) that have the most impact on whether the project will hit its deadline.

A typical example that's often brought up when explaining CPM is cooking. For a large breakfast, if you want your eggs, bacon, and toast to be ready to serve at the same time, you need to balance a bunch of competing priorities. The bacon, for instance, will take longer to cook than it will take for the bread to toast. Your toast has some slack, which means you can pop the bread in the toaster at almost any point during the bacon's cook time without delaying breakfast.

In this case, the series of tasks involved with frying bacon is the critical path to breakfast. The critical path determines how long it will take for you to cook and serve the food. Your toast and your eggs are still vital for a complete breakfast, but they have some slack (i.e., flexibility) for when you start and finish cooking them.

Projects that use critical path analysis (technically, CPM is an algorithm) tend to be way more complex than Saturday morning breakfast.
back in the 1960s when they faced interruptions in their factories. By identifying the critical paths in the production process, they could reduce the interruptions and speed everything up.

Since then, the idea of a critical path has developed into a project management mainstay, with many different industries using CPM for projects of all types. The broadness
for a critical path reflects its variety of uses: "The sequence of scheduled activities that determines the duration of the project."

These days, three of the most significant benefits project managers see from using the critical path method are:

Accurate time estimates and project optimization.
Project managers can use CPM to
how long a project will likely take. They can then optimize the critical path, thereby reducing the overall project's length of time.
Task prioritization.
With CPM, project managers know exactly which tasks absolutely
must
hit their deadlines and which tasks have some slack. With this information, they can prioritize what resources go where and when.
Risk management and workflow improvement.
In the process of creating a CPM plan, project managers can get the "lay of the land," visualizing potential bottlenecks, delays, and potential resources shortages before they arise.

What kind of projects is CPM good for?

Generally, CPM is good for large, complex projects with well-defined tasks. Constructing a house, for example, is an excellent CPM project because it requires you to manage a lot of competing priorities, but the core component tasks are straightforward: you need a foundation, walls, a roof, plumbing, electricity, and so on.

CPM is good for large, complex projects.

Large projects with many different teams, must-hit milestones, necessary deliverables, and interweaving task dependencies are great candidates for CPM. For projects like these, CPM provides a clear path toward on-time project completion. You might be juggling a lot of tasks, but you'll know which ones absolutely must hit their deadlines and which ones have a little wiggle room.

If your project is simple, CPM might not be the right tool. A simple project may call for a simpler strategy, like
.

CPM is good for projects with well-defined tasks.

CPM is best when you know what needs to happen to complete the project. If your project has the potential to change significantly throughout its lifecycle, consider a project management approach
.

How to use CPM in project management.

CPM has evolved into a fairly flexible framework, but it started in the early 1960s as a strict step-by-step process
. It's pretty dense, so we took the opportunity to de-jargon it a bit.
To use CPM in project management, you need to:

List out all tasks and assign dependencies.
Estimate time durations and slack time (also called total float) for each task.
Map out the tasks and identify the critical path.
Optimize your critical path.
Use your critical path to
as your project develops.

Here are the 5 steps in the critical path method in detail:

Step 1: List out all tasks and assign dependencies.

First, list out every task the project contains from start to finish. Traditionally, you then assign a symbol to represent each task (e.g., letters, numbers, or a combination) and use that symbol to denote predecessors and dependencies.

You may not need symbols if your project is small enough or if you're using
. Our
is a good example of a tool that doesn't require symbols. All that said, symbols can be handy for documentation and stakeholder communication.

Step 2: Estimate time durations in order to calculate slack time.

Alright, this is where the math comes in. Estimating time duration is pretty straightforward. Put in your best estimation for the amount of time it will take to complete each task.

Alternatively, many CPM templates (
) will take a cue from the PERT (Program Evaluation and Review Technique) strategy, which calls for three estimations: optimistic (O), pessimistic (P), and likely (M). With these three estimates, you then use the following formula to calculate likely task time duration (t).

(O + 4M + P)/6 = t

Once you have these estimations, it's time to calculate slack time, which is the amount of flexibility the task has before it delays the entire project. In the breakfast example, the cooking of the bacon has zero slack time because it takes the longest path to complete, while the toast may have a slack time of a couple of minutes.
There are five acronyms you need to know to calculate slack time:

ES = earliest start time
EF = earliest finish time
t = time duration
LS = latest start time
LF = latest finish time

First, you need to calculate the earliest possible finish time (early finish, or EF). To do so, pick the earliest time a task can start (early start or ES). If the task is dependent on another task, ES is contingent on the predecessor's EF. To calculate EF use this formula:

ES + t = EF

Pretty straightforward, right? Add your estimated time duration to the start date, and you get the earliest finish date.

Once you have ES and EF figured out, it's time to calculate the latest possible start time (late start or LS). To do so, pick the latest possible end date (late finish, or LF). Then use this formula:

LF - t = LS

Again, it's pretty straightforward. The latest time a task can start is the latest finish minus time duration.

With all this math complete for each task, you can calculate slack time. The formula for slack time is:

LS - ES = Slack Time

In other words, if ES = LS, there is no slack time, and that task is likely a critical task. If there is any delay with this task, the task preceding it, or the task-dependent on it, the whole project will be delayed.

Quick note: if you don't want to do all this math yourself, feel free to use our critical path method template, which automates a lot of this work for you.

👉


Step 3: Identify the critical path.

Lay out all your tasks as a Gantt chart using your estimated time durations and dependencies. Besides a Gantt flowchart, you can use a network diagram like the one Harvard Business Review uses above, but project management software these days will often create a Gantt chart automatically for you.

At this point, you might have a pretty decent idea of which tasks are critical and which aren't. Once they're all laid out visually, you'll be able to identify the critical path, which is the longest sequence of activities that determine the project's duration.

Keep in mind that sometimes there will be more than one critical path running parallel. This is not necessarily a bad thing, but you will have to prioritize two series of critical paths at the same time. It will be obvious with a Gantt chart if you have more than one critical path.

Step 4: Optimize your critical path.

In the process of calculating slack time and creating a Gantt chart, you may run into issues with your critical path activities. You may find that you have too many critical paths, and you need to adjust your LS or EF variables. Or, perhaps the tasks you created are too granular, leading to an overly complex critical path that's hard to manage.

It's perfectly fine for problems like these to arise at this stage of CPM.
is the time to fix issues before the project starts.

Step 5: Reduce risk as the project develops.

Once the project starts, all the CPM work you've done earlier on begins to pay off. Every project has its curveballs, missed deadlines, and lacking resources. With a CPM, you can prioritize which tasks get with resources when in order to complete the project on time. Tasks that have slack time are non-critical tasks, and it's alright to miss their deadlines as long as you're fast-tracking critical tasks. This management of the project process is how you reduce the risk of your project missing its deadline, going over budget, or creeping out of scope.

3 critical path method examples.

As we've covered already, many different industries use CPM for a variety of projects. The most common example you'll see around the internet is usually construction projects, but ad campaigns and research projects also make great examples.

Construction projects.

Construction projects have multiple teams and stages to coordinate along with
and a completion date. Knowing the critical path throughout the lifecycle of such a complex project will keep everything on track and inform all team members of what they need to prioritize.

CPM for construction projects example:

Advertising and marketing campaign launches.

Advertising and marketing campaigns are
because they often juggle many different channels, budgets, and assets at once. Not to mention that deadlines are paramount, especially around the holidays. A firm understanding of the critical path can help ensure everything finishes on time and is ready to launch.

CPM for advertising and marketing campaigns example:

Deep research and design projects.

If you're coordinating research or design across many different people, it can be challenging to keep track of what needs to happen by when. A critical path can keep everyone involved informed of the project's progress and what needs to happen to ensure completion time hits the mark.

CPM for research and design example:

Critical path method FAQ.

What are the advantages of CPM?

What are the disadvantages of CPM?

What is the difference between CPM and PERT?

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