Critical Path Method: An Introductory Guide for Beginners

Generally, a project is considered successful when it is delivered on time and on budget. In order to provide a realistic delivery date to the customer, it is important to know all the tasks essential to the completion of the project and their execution deadlines. The critical path method is a good way to estimate the end date of a project.

Definition

The critical path method, called Critical Path Method in English or CPM, was developed in the United States in the late 1950s, by Morgan R. Walker and James E. Kelley. However, it would have already been used between 1940 and 1943 for the Manhattan Project (the first atomic bomb of the Second World War).

The PMBOK (Project Management Body of Knowledge) defines the critical path as “a succession of planned activities that determines the duration of the project”.

The critical path method makes it possible to determine the total time required to complete a project based on the tasks essential to its completion, called critical tasks. To do this, you need to establish:

• a list of all the tasks required to complete the project,
• the dependencies between each task,
• the estimated time that it will take to complete each task (taking into account the deadlines imposed by the client).

Remember that you cannot start a task until the previous task is complete.

The critical path therefore corresponds to the longest succession of tasks dependent on each other (critical tasks). This defines the duration of the project. To succeed in the project, the tasks that make up the critical path must not be delayed, at the risk of delaying the entire project.

By knowing the critical path, the project manager can optimize his planning and assign his resources to the most important tasks first. Secondary tasks, which are not part of the critical path, can be delayed without impacting the project. If necessary, they can therefore be pushed back.

Today, the CPM method is used for all types of projects, ranging from software development to construction, including product development, research or even aerospace.

How does it work?

Visually, the critical path method is presented in the form of an arrow diagram. It takes place in 6 steps.

1. Identify all project tasks

Start by listing each task required to complete the project. Focus only on the most important tasks and do not consider secondary tasks or sub-tasks as this would make critical path analysis too complex.

For example, if your project is to build a house, the various tasks to be completed will be (approximately):

• find land,
• make the plans,
• dig the foundations,
• sink the slab,
• mount the walls,
• build the roof,
• do the exterior carpentry,
• do the insulation and the partitioning,
• install electricity, plumbing, heating,
• carry out the finishes.

2. Define dependencies between tasks

Once the task list is established, you realize that some tasks depend on the completion of other tasks. For example, you can't build your house until you find the land and you can't put up the roof until the walls are up.

You must therefore define the order in which your tasks are carried out and how they depend. To help you, ask yourself these three questions for each task:

• What task needs to be completed before it starts?
• What task must be completed at the same time as this task?
• What task should start immediately after it is complete?

3. Draw the diagram

Once you have prioritized your tasks and are aware of their dependencies, you can draw the critical path diagram by hand or using software. This diagram allows you to visualize at a glance all of your tasks and their relationship between them using arrows.

4. Estimate how long it will take to complete each task

Next, estimate how long it takes to complete each task. To help you, you can rely on previous projects and/or on the advice of experienced collaborators. Using the example of the house, ask the mason how long it will take to build the walls. Depending on the size of the project, the durations will be estimated in days or weeks.

5. Determining the critical path

Now, you can identify the critical path, that is, the succession of tasks that have the longest deadlines to complete. These are critical tasks. All of these tasks give you the total duration of the project. Make sure you trace the critical path visibly. Tasks that are not part of the critical path may fall behind schedule without impacting the project.

You can also determine for each critical task what are the earliest and latest start and end dates that will not affect the project schedule.

Attention, if you have several critical paths, this means that the critical path is likely to change once the project has started.

6. Update the critical path based on progress

The more your project progresses, the more you discover the real time required to complete each task. Take the opportunity to update your chart by replacing estimates with actual data. You then establish a new critical path that gives you a more realistic vision of the expected end date of the project. This way, you'll be more aware of whether you're on time, late, or early, allowing you to act accordingly.

Conclusion

The critical path method is one of numerous project management methodologies. One of its main advantages is to identify the important tasks of the project, those that will have an impact on the success or failure of the project. And above all, thanks to the critical path, you can visualize the progress of the project and you know if you are on time.

Note that for this method to be effective, you must not forget any task and clearly define their interdependence.