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Critical Path VS Longest Path in Project Management

April 2019

Critical Path

Critical path project management (CPM) is a technique used to complete projects on time by focusing on key tasks. One path through all the inter-connected tasks is the fastest avenue to take when completing any project. By focusing on the tasks that make up the critical path, the project manager maximizes the chances of completing the project on time.

Longest Path

The Longest Path is the longest continuous path of activities through a project, which controls project early completion. It is possible for otherwise defined critical path activities to not be on the longest path and longest path activities to not show calculated critical float.

Is there overlap between Longest Path and Critical Path?

Let say it this way – the Longest Path is a Critical Path, but not all Critical Paths are the Longest.

Because we can alter Critical Path through constraints, actuals, deadlines and other mechanisms, often we end up with a path that has more Critical activities than would be on the Longest Path.

If you start with a baseline schedule that has no actuals, no constraints and no deadline, and you perform some CPM scheduling on it, the path of 0 Total Float will be the Longest Path. If you add up the durations of all activities on that 0-float path, it will have the longest Total Duration.

If you calculate the Longest Path for a project that does have actuals, constraints, a deadline, etc, you will come up with a path of activities that are “important”, need I say “small-c critical” to the timely completion of the project.

Which one should I use?

I would strongly suggest you get to know your project’s Longest Path and monitor it. What the client wants to see might be something else, but the timely completion of Longest Path activities leads to your project finishing on time.

Most software packages let you track both Critical Path and Longest Path. Track both.

Lastly, I would advocate educating yourself on how the software that you use defines Critical Path and what sorts of things (constraints, relationships, lag,  etc.) will affect an activity’s Total Float.