Float is the amount of time that an activity can be delayed without affecting the project end date
Does that ring a bell from our discussion on Critical Path? We talked about how activities not on critical path can be delayed for a while without affecting the project end date. How much delay can the project absorb, that value is precisely known as float.
An instant corollary should be that float for activities on critical path should be zero as the critical path activities cannot be delayed at all.
What is Slack?
Now that you have a fair idea about Float, the next question should be about Slack. It is fair to ask that question given how these two terms typically appear together. The only reason for that is the fact that Float and Slack are essentially the same! Treat them as synonyms as far as Project Management and PMP is concerned.
Float is calculated by first finding out the critical path and then subtracting it with the next longest path. The result of this subtraction is the float value for all the activities on the aforementioned path. Note that float for critical path activities will always be zero even if they are common with the next longest (or any other) path.
Do refer to the video for step-by-step calculation of float in the example below,
Activity B has a float of 3 which means even if it gets delayed by 3 days the project end date won’t be impacted. Same goes for activities D, E and H.
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