- In this video, I want to answer the question, what is the critical path method? The critical path method is a project modeling methodology developed in the 1950s by Morgan Walker of Du Pont and James Kelley of the Remington Rand Organization. However, Kelley tells us that he attributes the origin of the term critical path method to a competing methodology, the program evaluation and review technique, PERT method, that have been developed at around the same time by Booz, Allen, Hamilton, and the US Navy.

These two methodologies are very similar. They use overlapping approaches but represent information in a different way. Indeed, the roots of the critical path method go back before the 1950s, into the 1940s, where they were used with some success on the Manhattan Project.

Of the two, the PERT method and the critical path method, it's the PERT method that is falling into less use these days. The primary difference is the PERT method shows activities on arrows, with nodes to represent how the activities connect to one another. The critical path method, on the other hand, shows activities as nodes, usually represented by boxes with arrows to show the logical links between them.

The other principal difference between the PERT and critical path methods is that the PERT method uses three point estimates for the durations of activities, whereas the critical path method more simply just takes a single point estimate.

To apply the critical path method, we take a simple six step process. In step 1, we specify each activity, and we usually draw these from our work breakdown structure.

In step 2, we start to sequence the activities by understanding the dependencies that link one activity to the next. This enables us in step 3 to draw our network diagram. This is a graphical representation that uses arrows to connect the activities, which are shown as boxes. And in the boxes, we can record the information that is important to us.

The next step, step 4, is to estimate how long each activity will take, the durations. And as I say in the critical path method, traditionally, they're single point estimates, which is both the strength, because of the simplicity, but also the weakness, because of the lack of recognition of contingency and variability of the critical path method.

The fifth step is to identify the critical path. And the critical path is the longest route through the network. It's critical because any extension to the duration of an activity, because of a delay, will necessarily lengthen the critical path and therefore delay completion of the project.

And finally, step 6 is to use your critical path as a monitoring tool as well as a planning tool, and to update it to show progress. So the critical path is the longest route through the network. But we also calculate the earliest and the latest possible start times for activities.

These enable us to understand where we have float, the ability to vary the start time of an activity without affecting the completion of our project. Float is sometimes known colloquially by project managers as slack in the plan.

Of course, a project can have more than one critical path. Indeed, if every route through the network is the same duration, one could argue that the project has been optimized. There is maximum risk. However, all resources are being fully deployed.

Note, however, that if you have parallel paths through the network, you can only make this work if you have the resource flexibility to ensure that resources are deployed according to your plan.

So the critical path method is a really strong way to understand the logic of your project and a great way to calculate the duration. But I have to say that these days, far more project managers prefer the gantt chart.

But the truth is the gantt chart and a critical path network diagram our siblings, and they are simply two different representations of the same information. And high end project planning tools are adept at taking a base set of information and representing them either as a critical path network or as a gantt chart. The critical path method, then, is just one way to plan, display, and track the progress of your project.