A beginner’s guide to the critical path method in project management

Large and complex projects require the use of many materials and working parts. Without a clear plan for undertaking such a job, it’s easy to become overwhelmed and skip important tasks. When this happens, your project is less likely to meet critical deadlines. Or it could fail completely.

Using project management methods that break large jobs down into smaller tasks can help ensure your project goes to plan. The critical path method (CPM) is one such project management technique.

CPM is used to identify the vital aspects of any project and its dependent factors to define tasks and ensure the project is completed accurately and on time.

Read on to find out exactly what CPM is and how to use it. You’ll also find some examples of successful use cases.

The critical path method is a project management method used for calculating, planning, and scheduling large or complex projects. It enables project planners and teams to create a clear roadmap of the jobs or activities that are critical to completing a project on time. By identifying and scheduling fundamental tasks, project managers can assign a specific job’s start time, expected duration, and finish time.

Every project contains vital tasks that determine whether it’s completed on time. If any of these tasks fall behind schedule, so does the whole project.

CPM is a step-by-step plan for identifying critical tasks and creating a clear timeline (or path) in which these tasks will be completed. The schedule of these core project steps is the critical path. Once these tasks are scheduled, you can add other (less crucial) activities to the timeline.

What is a critical task?

A critical task is a job or activity central to a project’s timeline. Other activities will also be delayed if these central tasks are not completed on time.

Every project includes core tasks that must be finished before dependent tasks can begin. For example, you can’t put a roof on a home if the frame hasn’t been erected. In essence, critical tasks are the activities that must be carried out as planned to avoid disrupting the project timeline.

Why must non-critical tasks be monitored?

Critical tasks aren’t the only tasks you’ll need to take care of to complete an entire project. You’ll need to carry out other jobs, too. However, these can be executed less urgently. Since they aren’t core steps, you can choose to delay them until they fit into the schedule better.

By monitoring non-critical tasks alongside the undertaking of critical tasks, you can make informed decisions about when each task should be completed. This approach enables teams to complete several tasks simultaneously without critical delays.

Where was CPM first used?

The critical path method isn’t new. The earliest recorded use of CPM occurred in the 1950s when two unrelated projects were experiencing unexpected delays.

While the US Navy’s Fleet Ballistic Missile (Polaris) Program was running behind schedule, the chemical company DuPont faced delays in plant turnarounds. Those running these projects needed a technique to get them back on track and deliver them as planned.

The developers of the Polaris Program created a solution called the project evaluation and review technique (PERT). DuPont developed the critical path method.

The PERT method calculates three different time estimations to determine the most accurate duration of each task. DuPont’s CPM plan uses a single time duration for the entire task to measure extra costs that would accumulate if the project duration is decreased.

Why is CPM important in project management?

CPM creates a project roadmap that establishes the longest sequence of tasks that must be executed to complete an entire project. By defining core tasks and the tasks that depend on them, project managers can outline the timeframe in which the project can be successfully executed. This helps reduce noise and align priorities, enabling multiple teams to complete tasks simultaneously without delays.

When should you use critical path analysis?

Critical path analysis is a way to refer to the early stages of CPM. CPM typically aims to develop an accurate project schedule, so the process occurs before the project’s start date—perhaps during the project’s planning phase or even during the process of developing a bid or project proposal.

Understanding a project’s critical path can help project managers estimate costs and resources and develop a predictable schedule.

Virtually every project is planned based on completing the job as quickly and cost-effectively as possible without compromising quality. This approach offers the best return on investment (ROI) and ensures minimal resource waste. You can use CPM to make informed decisions about the best way to achieve this goal.

To calculate the critical path, start by identifying all tasks in the project. Once you have a clear idea of everything that needs to be done, you can assign levels of urgency based on dependencies and the estimated time needed for each task.

Take these steps to calculate your project’s critical path:

1. Define all the tasks required to complete the project.

2. Define the time requirements for each step based on existing knowledge or industry standards.

3. Establish dependencies (tasks that must be completed before another specific step can begin) for each task.

4. Identify which activities must be completed on time to avoid delays due to dependencies. These are your critical tasks.

5. Align critical paths in a sequential timeline with assigned start and finish times.

The significance of float or slack in CPM

Projects are rarely executed exactly as planned. Delays, resource constraints, unexpected incidents, and scope creep create conditions that cause any task to take longer than planned. To prepare for such issues, project managers should consider the consequences of delays.

The time a task can be delayed for without affecting dependent tasks or delaying the overall project is referred to as float or slack.

Activities on the critical path have zero float. If a critical task is delayed, it will immediately affect dependent tasks and the completion of the project. However, many other tasks can have varying levels of float.

High-float tasks can be delayed with minimal consequences. They simply need to be completed before the project deadline or before they affect another non-critical task. Label high-float tasks as low priority so that you focus on assigning resources to critical tasks. When team members become available, they can take care of high-float tasks while other tasks progress simultaneously.

Identifying slack for each task lets you assign start times and juggle tasks without affecting the project’s timespan.

Large and complex projects can take weeks, months, or even years to complete. Long time frames combined with project intricacies may create unexpected changes that force you to expedite some tasks and postpone others.

To take these contingencies in your stride, you’ll need methods in place to address how such issues impact the overall project timeline.

The two most common methods of dealing with contingencies and constraints when using CPM are fast-tracking and crashing.

Fast-tracking

Fast-tracking is easy to understand if you’re used to multi-tasking to stay on schedule. When deadlines are looming, fast-tracking is the process of executing multiple jobs on the critical path simultaneously. This approach requires you to dilute resources to complete multiple jobs at once and can only be used for activities without dependencies.

Crashing

When you need to meet updated or emergency deadlines, crashing is the process of allocating more resources to speed up activities. You might bring in more professionals or use resources from tasks with high floats.

Crashing is typically used in emergencies because it can impact critical tasks and project scope. When the project scope is affected, you’ll need to inform all project stakeholders.

CPM isn’t the only project management method for scheduling and prioritizing project tasks. Comparable methods are used to evaluate the duration and dependencies of specific tasks. While these methods have similarities, their overarching goals differ.

Critical path method vs Gantt charts

Gantt charts map out projected activities and track them against a set timeline. Like CPM, they show dependencies between tasks, and you can use them to create a timeline. However, there are distinct differences between the goals of CPM and Gantt charts and how they are displayed.

• CPM is displayed as a network diagram, while Gantt charts are displayed as a horizontal bar chart.

• Gantt charts show the resources required for each activity.

• Gantt charts plot activities on a timescale, while CPM plots them on a network diagram with an estimated time duration for each task.

• CPM identifies critical and non-critical paths and calculates project duration. Gantt charts are designed to visualize how project activities are progressing.

Critical path method vs PERT

The project evaluation and review technique was developed around the same time as CPM to solve similar issues, but the techniques are quite different.

CPM and PERT are both used to evaluate project duration. They identify dependencies and provide an estimated duration for each task. However, they have several notable differences in execution.

• PERT is used to estimate the time required to complete activities, while CPM is used when task durations are already estimated.

• CPM is designed to evaluate time–cost trade-offs and establish priorities, while PERT focuses on meeting deadlines or shortening a project’s duration.

• PERT has three estimates for each activity, while CPM only has one.

• PERT is often used for projects that have significant uncertainty, while CPM is better suited to projects where tasks are well-defined.

The critical path method is an excellent project management tool to help you deliver your project on time and within budget. It can be useful in recognizing potential delays and scope creep while creating a clearly defined schedule for the project.

Below are some of the most common advantages of using CPM for large and complex projects:

Helps prioritize tasks

CPM’s overarching goal is to identify essential tasks and ensure their timely completion.

Identifying the critical path enables you to clarify which tasks depend on one another, establishing the order in which they must be completed.

Once the critical path is defined, you can prioritize the non-critical tasks based on their slack level. The approach eliminates the “noise” in a multi-faceted project and keeps teams on track.

Improves team communication

Large projects typically require the participation of multiple teams with different specialties. These teams work together to establish an accurate critical path. When the resulting CPM diagram is used as a single source of truth, every participant in the project will have similar expectations of how the work schedule will play out.

Optimizes efficiency

CPM is used to identify the earliest possible starting point and the latest acceptable finishing point for each project task.

Mapping out a schedule with accurate timeframes and dependencies can enable project managers to identify areas that could shorten the critical path. You might achieve this by assigning more people to the job or juggling non-critical tasks for optimal efficiency.

Helps create accurate schedules when paired with PERT

PERT and CPM have distinct differences that naturally complement each other. While both techniques are designed to evaluate time, PERT is focused on time management, while CPM addresses how the project timeline affects the budget.

PERT charts contain three estimates for each task, making it easier to discover potential risks and avoid delays.

Helps map out project plans when paired with Gantt

Gantt charts plot project tasks on a time schedule in a horizontal graph, which acts as a visual communication tool. They can be used to track actual progress against planned progress to help maintain an accurate schedule.

When combined with CPM, Gantt charts give you a more detailed view of the product path, including the required resources for each task.

Now that you have a clear understanding of the CPM’s purpose, you can apply the process to devise an accurate project schedule. Follow the steps below to plan your project using the critical path method:

1. List project activities

Begin by evaluating your complete project and breaking down the necessary work into manageable steps. Create an ordered list of all the tasks required to complete the job.

2. Identify task dependencies

Some tasks can’t begin until others are finished. Examine your list of tasks and define each activity’s immediate predecessor. Some tasks won’t have dependencies, but critical tasks often do.

3. Create a critical path diagram

Identifying dependencies enables you to create a sequence of events. Create a chart that maps out the tasks in the order they should be completed. The network diagram should include arrows or symbols to identify dependencies.

4. Estimate the timeline for each activity

Estimate the time required to complete each task on the diagram using prior experience or industry knowledge.

Use the PERT method to accurately calculate the best-case estimate, the most likely estimate, and the worst-case estimate. These three values can be used to determine slack more accurately and develop a precise timeline.

5. Use the critical path formula

Once you’ve established estimated durations for each task, you can develop a timeline for the entire project using the critical path algorithm. The critical path formula uses a forward pass and a backward pass to establish a timeline.

The forward pass uses a specified start date as the earliest start (ES) for the first task and calculates the earliest finish (EF) by adding the estimated duration. The process proceeds through the schedule by defining the ES as the highest EF of immediate predecessors.

The backward pass calculates late start (LS) and late finish (LF) dates using the lowest LS value from immediate successors to calculate LF. This pass starts with the last scheduled activity and moves backward through the schedule.

Here’s a simple example using a forward pass and backward pass to establish a timeline for a project with three activities: A, B, and C. The numbers represent the duration of each activity in days. Each activity is dependent on the one before it—B is dependent on A, and C is dependent on B.

Here are the activity durations:

• Activity A: 3 days

• Activity B: 2 days

• Activity C: 4 days

Forward pass

Start at the beginning. Set the ES for the first activity to 0.

• Activity A:

  • ES = 0

  • Duration = 3

  • EF = ES + Duration = 3

• Activity B:

  • ES = EF of A = 3

  • Duration = 2

  • EF = ES + Duration = 5

• Activity C:

  • ES = EF of B = 5

  • Duration = 4

  • EF = ES + Duration = 9

The EF of C is the project duration in this case.

Backward pass

Start at the end. The project duration is the LF of the last activity.

• Activity C:

  • LF = EF of C = 9

  • Duration = 4

  • LS = LF - Duration = 5

• Activity B:

  • LF = LS of C = 5

  • Duration = 2

  • LS = LF - Duration = 3

• Activity A:

  • LF = LS of B = 3

  • Duration = 3

  • LS = LF - Duration = 0

You now have the ES, EF, LS, and LF for each activity.

Critical path

The critical path is the path with zero slack (total float). Activities on the critical path have the same ES as LS and the same EF as LF.

In this example, the critical path is A -> B -> C. The project duration is nine days, and any delay in activities on this path will directly affect the project completion date. Activities A and C are “critical” in the sense that they must be completed on time to avoid delaying the project.

6. Identify the critical path

Define critical tasks as those with no slack level (zero float) and align these tasks into a schedule based on immediate dependencies. Assign a time duration to each critical task derived from the formula used in the previous step. The activity sequence with the longest duration is the critical path.

7. Revise during execution

While the critical path is determined during the planning phase, it shouldn’t be set in stone. Update the critical path chart or diagram as you execute the project. As changes occur in the project schedule, you can define ways to address delays. You can develop a more accurate schedule by updating it to account for constraints and contingencies.

The critical path method can be used to manage nearly any type of project in which you can estimate the duration time of specific tasks. Observing examples of CPM in practice can help you clearly visualize how the technique is used.

CPM in construction

Building a house is one of the most common examples used to clearly illustrate project planning with dependencies and potential time constraints. Construction is a complex project with many dependencies.

Activities

• Conduct excavation

• Build a foundation

• Construct a wooden frame

• Lay brickwork

• Box in the walls

• Build the roof

• Wire electricity

• Install plumbing

• Install heating and ventilation

• Lay flooring

• Finish wall carpentry

• Fasten gutters and downspouts

• Install plumbing fixtures

• Finish interior carpentry

• Paint

• Put gutters and downspouts in place

Identifying dependencies

Construction involves many dependencies.

The core jobs will outline your project’s critical path. The process of pouring footers and building a foundation is dependent on the starting task—excavation. Similarly, framing is a critical task that must be completed before walls can be constructed and the roof can be built. However, the brickwork and the electricity can be reasonably postponed until a convenient time.

Jobs with float can be executed in parallel to dependent tasks. For example, an available worker can lay brickwork while others are building the roof.

Therefore, jobs on the critical path are most likely to be excavation, foundation, framing, roof, rough wiring, and rough plumbing. The other tasks in the project will have some float and can run parallel to other tasks.

CPM for shorter tasks

It sometimes helps to consider how project management works from a simpler perspective. If you’re a practiced cook, you’re accustomed to the process of taking care of multiple tasks while being mindful of dependencies. Yet, an inexperienced cook could forget essential steps.

Imagine preparing breakfast for friends. You plan to cook pancakes, eggs, and bacon. You’ll prepare many of these foods in tandem, but some steps have dependencies.

Activities

• Purchase eggs, bacon, flour, milk, maple syrup, oil, and butter

• Preheat the oven to 200 degrees to keep the food warm until serving time

• Measure and mix the flour, eggs, oil, and milk to create pancake batter

• Fry the bacon

• Cook the pancakes

• Season the eggs

• Fry the eggs

• Keep the pancakes warm in the oven while you cook the other foods

• Top the pancakes with maple syrup

Identifying dependencies

Again, your starting step for this project is a critical task that must be completed before any other activities can begin: purchasing the ingredients.

Moving forward, you can’t cook the pancakes before mixing the batter. However, since you plan to keep food warm in your preheated oven, you can fry the bacon while you’re mixing the batter or flipping the pancakes. Ideally, you would prepare the eggs last so they will be at the optimal temperature when the meal is served.

The jobs on the critical path are most likely to include buying ingredients, preheating the oven, mixing the batter, and frying the eggs. Other tasks can be completed in parallel to these essential steps.