The critical path is the chain of work that sets the finish date: delay it and the finish moves. Finding that chain sounds simple until constraints, calendars and long lags get involved. Then the filter called Critical stops telling the whole story.
Total float and free float
- Total float: how long an activity can slip before it moves the project finish or a constrained date.
- Free float: how long it can slip before it moves any successor.
- Near-critical work: low float, but positive. Today's near-critical path is often next month's critical path, so watch it too.
Why the critical filter can mislead you
Total float is measured against the latest allowable finish. Add a hard constraint and float is measured against that date instead, so the critical filter may show activities that do not drive the project finish, and hide those that do. Multiple calendars do the same in a quieter way, because a day of float on a five-day calendar is not a day on a seven-day calendar.
Trace it instead
Start from the activity that sets the project finish and walk backwards through its driving predecessors, one link at a time. If the chain reaches the data date, you have a real path. If it stops, the place where it stops is the finding: usually a constraint, or a missing link.
Do this after every update, and compare it with the last one. A critical path that jumps to a different part of the job is the most useful thing an update can tell you.
See driving-path tracing in the logic viewerIs your own schedule healthy?
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