glossary — plain english
Slack (float)
Also called: float, total float, free float
How long a task can be delayed without delaying a later task (free float) or the project's finish date (total float).
Suppose a one-week photo shoot starts alongside a three-week build, and both must finish before QA begins. The photo shoot can be delayed by up to two weeks without delaying QA. Those two weeks are its slack.
Free float measures the room before a delay affects the next task. Total float measures the room before it affects the project finish. Tasks with no total float are on the critical path; checking this helps you decide which delays need immediate attention.
Slack comes from a task's position in the schedule. A buffer is time deliberately added to absorb delays. Both provide room to move, but they arise in different ways.
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