Construction Scheduling. What are the Critical and Near Critical Paths? Why do we care?

When the schedule is developed, the schedule logic should be complete. All activities should have at least one predecessor and one successor, with the exception of the first and last activities for the schedule.

If this is done, the combination of activity durations, activity calendar assignments, and schedule logic create the CPM schedule network. This CPM schedule network is made up of many sequences (logic paths) of activities.

In simple terms, the longest continual path from the project start to the end of the project or the next hard constraint establishes the Critical (Longest) Path.

The Total Float values are a just the product of the forward and backward pass through the CPM schedule network. Just because the Total Float value is zero does not make the activity critical.

However, the Total Float values are important. If the Critical Path has a Total Float value of zero, then the Near Critical Path activities will most likely have a Total Float value very close to zero. I say most likely because the activity calendar assignment has an impact on the Total Float values. But often, this is a simple rule of thumb for determining the Near Critical Path. I like to keep an eye on work that is on the Critical Path first and then also pay attention to work that falls on the Near Critical Path(s) within one or two weeks of the Critical Path for large projects and a few days for small projects.

It’s not unusual for the PM and Superintendent to focus on work on the Critical Path and not pay enough attention to work on the Near Critical Path(s) and then have the Critical Path shift because a lack of progress for the Near Critical Path work took over the Critical Path.

This is just one reason complete logic, calendar definition and assignment, and limiting constraints are important when developing a baseline CPM schedule. If you scatter constraints throughout the schedule, you will not have a true Critical Path for the project. Without complete logic, you do not have the means to accurately complete the forward and backward pass that establishes the CPM schedule network scheduled dates and Total Float values. Without calendar definition and proper assignment to activities, durations will be erroneous and this will ultimately produce a false schedule network.

I know many of you can offer additional comments and recommendations. I welcome your comments and input. My goal, as always, is to help our industry and help the projects we support….

I’d love to hear what you think!

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Paul Epperson CCM, PMP, PSP, PMI-SP