Critical path method

What it means

The critical path method is a project scheduling technique that finds the longest sequence in a network of dependent tasks, the critical path. Its length sets the shortest possible time in which the project can be completed if everything goes to plan.

Tasks on the critical path have no slack. Any delay to one of them pushes back the end of the whole project. Other tasks have float, meaning they can slip by a certain amount without endangering the deadline until that float is used up.

The method was developed in the late 1950s to manage large industrial projects and is still part of most project management tools. The result is often shown in a Gantt chart with the critical path highlighted.

How it works in practice

The process starts by breaking the project into tasks and setting their durations and dependencies, that is, which task must finish before the next can start. From this the earliest and latest possible start of each task is calculated, and the difference shows the float.

The project manager focuses mainly on tasks on the critical path and their inputs. In an office move these might be furniture delivery and internet connection; in a switch to a new accounting system, data migration and testing before the new year begins.

If the project needs to be shortened, it only makes sense to reinforce critical path tasks, for example by adding people or running work in parallel. Speeding up tasks off the path does not bring the deadline forward. Such decisions affect cost and scope, the trade-offs within the project management triangle.

What to watch out for

The critical path changes during the project. When a task off the path slips by more than its float, it becomes critical. The plan should therefore be updated regularly against actual progress, not drawn once at the start and relied on thereafter.

The method assumes that task durations are known and resources are available. In practice people are shared across several projects and estimates tend to be optimistic. Planning should take capacity planning into account and build a sensible buffer into key milestones.