Jory MacKay on January 03, 2023 How to Plan Successful Projects With The Critical Path Method
For most project managers, it’s not the beginning or end of a project that’s difficult to plan for - it’s the “messy middle.” Even with the best scope of work and an actionable project plan, it’s impossible to know what might happen once you start working towards your goals.
The critical path method - commonly known as CPM - is one of the best ways to remove uncertainty and clearly identify and plan out the essential steps of your project.
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It can be a lot of work to plan out your critical path, but the result is often less risk and time wasted on non-value-adding tasks, and ultimately a greater chance of project success.
In this guide, we’ll explain the essential steps of the critical path method, what makes it so effective, and how to use it on your own projects.
What is the critical path method (CPM)?
The Association for Project Management (APM) defines the critical path method as:
“A technique used to predict project duration by analysing which sequence of activities has the least amount of scheduling flexibility.”
But, what does that really mean?
In simple terms, the critical path method is a technique used by project managers to identify and schedule the most time-critical tasks for their projects. This is essential because if any critical tasks are delayed, the whole project will be delayed, too.
For this reason, project managers have used CPM for over 50 years to manage project risk by identifying the tasks that need the most attention and oversight.
It’s not the beginning or end of a project that’s difficult to plan for - it’s the “messy middle.”
Famously, CPM has been used to plan and deliver projects such as the World Trade Center and the Manhattan Project, and has even been featured in fictional books such as Odds On by Michael Crichton.
But CPM isn’t just a fancy planning technique.
The critical path method delivers a whole host of benefits, such as:
- Fewer bottlenecks. CPM helps project managers identify the most time- and effort-intensive project tasks. If you know where the bulk of the work is, you can make a considered effort to avoid bottlenecks.
- Better understanding of dependencies. All projects have dependencies, but some matter more than others. CPM helps you identify the critical ones to your progress, allowing you to give them the attention they need.
- Clear priorities. Once you know where the bottlenecks and the dependencies are, you and your team can prioritize accordingly.
- Better resource management. In a recent survey, over 60% of project professionals agreed resourcing is a huge problem. CPM helps you understand your resourcing needs by clearly showing which tasks need tackling and when.
- Improved future planning and product roadmap accuracy. Knowing the critical work you need to achieve allows you to better plan your long-term objectives. This is especially important in product strategy management as a more defined roadmap helps you set, manage, and exceed stakeholder expectations.
The essential elements of the CPM (+ definitions)
Before diving into the somewhat complicated process of critical path analysis, it’s important to first understand the main elements that underpin CPM.
When defining your CPM, you’ll need to understand the following terms:
- Work breakdown structure (WBS): Given that CPM is a task-based scheduling technique, you need to fully understand the work ahead of you before you can plan it. A Work Breakdown Structure helps you systematically break down the tasks you need to complete, giving you that truly granular view of your project.
- Task Duration (D): Once you have your tasks broken down, you need to estimate how long each task will take. Estimating projects is a tricky thing, but there are many estimating techniques out there, including comparative, reference class, and 3-point.
- Dependencies: Effective task management is all about dependency identification and management. For CPM, you’ll need to identify which tasks can’t start until another one finishes. E.g., you can’t brush your teeth until you’ve put toothpaste on your toothbrush.
- Timings: As you develop your CPM, you’ll see four common abbreviations when it comes to scheduling — ES, LS, EF, and LF. Here’s what they each mean:
- Earliest Start (ES): The earliest a task can start.
- Latest Start (LS): The latest a task can start.
- Earliest Finish (EF): The earliest a task can finish.
- Latest Finish (LF): The latest a task can finish.
- Float/Slack/Buffer (F): As you begin scheduling your CPM, you’ll begin to see which tasks have some wiggle room. Depending on where you are in the world, this is referred to as the tasks’ Float, Slack, or Buffer. For this article, we’ll use Float (F).
Creating your WBS, calculating your task durations, and identifying dependencies are key prep work items for your critical path analysis and will be set up like this:
On the other hand, the exact task timings and float are worked out as part of the CPM process.
It’s key to remember that CPM helps you identify your critical tasks, you won’t know which tasks are critical beforehand. That’s because no two projects are the same. The same task can be critical in one project, but not necessarily critical in another. It all depends on the unique scheduling of that project.
To bring CPM to life, let’s move on to look at how you can execute your own critical path analysis.
The Critical Path Method isn’t just a fancy planning technique.
How to identify your project’s critical path (6-step guide)
The good news is that identifying your project’s critical path isn’t rocket science. If you follow these six simple steps, you’ll be able to create your own critical path in a matter of hours.
1. Create your Work Breakdown Structure
To adequately plan the project you first need to understand all the work and tasks required to deliver it. This granularity will help you map out your critical path.
How to do it:
- The best way is to get your project team together for a workshop to brainstorm how your project breaks down.
- To make it easier, try first breaking the project down into phases, milestones, or waves. Then, logically identify the individual sub-tasks.
Practical example:
John is Project Manager at Web4U, a Website Development Agency. John is given a new project to build a website for a client. He starts by building a Work Breakdown Structure, breaking down the tasks he’ll need to complete to deliver the project. To make it easier, he divides the work into three phases: Design, Build, and Test & Launch.
2. Estimate your task durations
How will you ever know the duration of your project if you don’t know how long each work item takes?
Understanding your task durations (D) will allow you to map your critical path and understand where the pinch points and bottlenecks may be.
How to do it:
At this stage, it’s all about asking the experts and learning from people who have been where you are now. Here are a few different ways you can properly estimate task duration:
- Use an estimating technique such as comparative, reference class, and 3-point.
- Speak to people who have done a similar project before to get their opinion on how long each task will take.
- Get online and research past projects and industry best practices.
Pro tip: Especially in Government projects, there are often great resources online that explain the estimating techniques behind a publicly funded project.
Practical example:
First, John puts each task into a table, giving them a unique ID number. He then estimates how long each task will take to complete in the number of days. John is an experienced web development Project Manager, so he uses data from previous projects to estimate this one.
3. Identify task dependencies
Your most critical tasks will be the tasks that either enable or block progression. So, you need to understand where the dependencies lie. Once you do this, your end-to-end scheduling will come to life.
How to do it:
- Get together as a team to discuss the flow of work. Take each task one by one, and consider what needs completing for the next one to begin.
- Speak with others who have done similar projects to identify the dependencies they had during their delivery.
- For each task, create a critical path planning box and link the dependencies together.
Practical example:
John sits down with another Project Manager, Sarah, at Web4U and brainstorms the dependencies for his project tasks.
Even though John has completed many projects before, Sarah has more recent experience, so she provides some input. They agree on the project dependencies and map them against the tasks in the critical path planning boxes.
4. Map your tasks (forward pass)
Now that you know the tasks, durations, and dependencies, it’s time to plan them formally. This will give you your project schedule and begin to show how and when you’ll get from start to finish.
How to do it:
Begin with your ‘Forward Pass’, inputting the following for each task:
- Earliest Start (Top left)
- Duration (Top middle)
- Earliest Finish (Top Right – Earliest Start + Duration)
Complete these boxes for all tasks from start to finish. As you input your data, remember these critical facts:
- The first task always starts at Day 0.
- Where a task is dependent on another, the task’s Earliest Start will be the Earliest Finish from the previous task.
- Where a task depends on more than one other, the Earliest Start is the highest Earliest Finish from all others.
Practical example:
John begins mapping out his schedule by completing his forward pass. Task A starts on Day 0, with the durations moving forward for each task. At Task E, John takes the highest Earliest Finish (day 15 from Task D) to account for the double dependency.
5. Review your tasks (backwards pass)
Once the forward pass is complete, you need to head backwards to fully assess the criticality of each task. This is the stage where the critical path really comes to life, and you’ll be able to see the key differences between tasks.
How to do it:
To begin, start with the final task in your schedule. The Earliest Finish (EF) becomes the Latest Finish (EF), as you can’t finish at two different times.
Next, work backward in the opposite way you did for step 4, filling out the following boxes as you go:
- Latest Finish - Bottom Right
- Duration - Top Middle (this won’t change from step 4)
- Latest Start - Bottom Left (Latest Finish - Duration)
Complete these boxes for all tasks from start to finish. As you go through them, remember these critical facts:
- Where a task depends on another, the task’s Latest Finish will be the Latest Start from the previous task.
- Where a task is dependent on more than one, all of the pre-requisite tasks use the same Latest Start and their Latest Finish.
Practical example:
John completes the backward pass for his project tasks. For this, he starts by taking the Earliest Finish (EF) from Task F and making it the Latest Finish (EF). From there, he subtracts backward. Where Task E has two pre-requisite tasks, both Task C & D take the same value of 15.
6. Calculate float and identify your critical path
With your task, dependencies, and durations mapped out, it’s now time to identify whether each task has a float. Crucially, the tasks with no float have no room for error, and as such, they’re your critical ones.
How to do it:
- Identifying the float is easy. Simply subtract the Latest Start – Earliest Start or the Latest Finish – Earliest Finish. Both should equal the same number. If they don’t, you’ve made a mistake on your forward or backward pass.
- As you review, enter each number in the Float box.
- Your critical path is the sequence of tasks that have a 0 in the float. That’s because if any of these tasks are delayed, the only outcome will be for the entire project to be delayed.
Practical example:
Now that John has completed his backward pass, he calculates the Float for each task. John realizes only one task, Task C, has any Float, meaning Tasks A, B, D, E, and F make up his critical path. John now knows which tasks are critical, and if any of them are delayed, the entire project will be too.
Examples of how to use CPM on your projects
CPM is a powerful tool for helping you identify the risky tasks in your project. Here are some other key ways CPM can help you and your project succeed.
- Identify compression opportunities. If you’re under pressure to deliver quickly, CPM can help you identify the tasks that, if done faster, could shorten your project timelines.
- Resolve shortages in resources. CPM gives very clear visibility on the tasks that matter, helping you see where resources are needed most. If you’re starting to fall behind on your timelines, CPM will tell you and highlight where to put your resource to get back on track.
- Collecting data for future projects. Because CPM is such a detailed planning method, it’s a great data point for future projects. If others are doing a similar type of project in the future, it clearly shows how projects have been broken down, how they’ve been estimated, and where the dependencies are.
- Prioritising projects and portfolios. CPM helps project and program managers prioritize workloads so they can clearly see the effects a delay would have. This is especially important when projects are sharing resources or are dependent on each other.
- Escalating issues. CPM provides a very clear view of how delays can affect your project timelines. This can help you raise issues with stakeholders or sponsors, as you don’t need to be a project management expert to understand a CPM map.
The Critical Path Method is one of the best ways to remove uncertainty from your project.
Critical path method (CPM) vs. PERT vs. Gantt charts
Critical Path Method isn’t the only way to estimate projects. PERT and Gantt are two of the other most popular scheduling techniques available to project managers.
Let’s look at how they compare to CPM.
PERT vs. CPM
What is PERT?
Program Evaluation Review Technique (PERT) is a planning tool used to calculate the time it will take to complete a project when the task durations are unknown. With PERT, project managers create three different estimates of their project based on:
- The shortest possible amount of time each task will take
- The most likely amount of time each task will take
- The longest amount of time each task will take
Then, working backward from a fixed point, PERT is used to determine if a project is achievable and the most likely timeline.
What’s the difference between PERT and CPM?
PERT is often used when project managers don’t know how long tasks will take, or they need to work backward from a fixed delivery deadline.
While PERT charts look similar to CPM, the CPM technique is used to schedule known project tasks and surface any critical dependencies by identifying float.
Gantt charts vs. CPM
What is a Gantt chart?
The Gantt chart is the most popular planning tool used in project management, showing tasks displayed against time. Typically, tasks are listed on the left, with task bars going left to right to show their sequencing, start and end dates, and duration.
What’s the difference between a Gantt chart and CPM?
Typically, Gantt charts don’t show how different tasks and activities relate to one another, instead, they simply show how they happen over time. This is the key difference to CPM, where a project manager can identify their project’s critical path of interdependent tasks.
Let Planio help you stay focused on your most critical tasks
The Critical Path Method is one of the most commonly used planning tools, helping project managers identify the tasks that really matter. Understand those tasks, and you’ll be able to prioritize better, solve problems faster, and use your resources more efficiently.
And while many think that critical path analysis is hard, if you link it with other project techniques, such as a Work Breakdown Structure, it’s easy to granulate, define, and schedule your project’s work.
And if you need an extra helping hand, tools like Planio are ready to become the center of your project planning universe. With features for creating, linking, and mapping tasks, Planio helps you gain visibility of your work and bring your entire team onto one page.
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