Schedule Mechanics & Data
Near-Critical Activity
A near-critical activity has low but positive total float -- close enough to zero that a small additional delay would push it onto the critical path.
Definition
There's no universal float threshold that defines "near-critical" -- it's typically a project-specific judgment call, often somewhere in the range of a few days to a couple of weeks of total float depending on the project's size and volatility. What makes near-critical activities worth tracking specifically is that they're the activities most likely to become critical between one update and the next; watching float erode toward zero on a near-critical activity is an early warning sign that it may soon start actually driving the finish date, even though it technically isn't yet.
Example
An activity with 4 days of total float on a two-year project isn't on the critical path today, but a single-week slip on any of its predecessors would push it there -- exactly the kind of activity worth watching closely between updates.