In project management, the terms slack and float describe the length of time that an activity can be delayed without delaying the finish date of a subsequent activity, or the finish date of the entire project. The terms are most commonly applied to a network analysis technique, known as the Critical Path Method, which was developed by the DuPont Corporation in 1957.
TL;DR (Too Long; Didn't Read)
Float and slack in project management refer to how long you can delay a project's activity without compromising the finish date of another activity or the project as a whole.
Slack Versus Float
The terms "slack" and "float" are often used interchangeably. However, the main difference between float and slack is that slack is typically associated with inactivity, while float is associated with activity. Slack time allows an activity to start later than originally planned, while float time allows an activity to take longer than originally planned.
The Critical Path Method
The Critical Path Method depicts a project as a network diagram, in which each node on the network represents an activity. The nodes are joined together by lines, or arcs, which represent the events that mark the beginning and end of each activity.
The critical path is the longest path through the network diagram and determines the shortest time you need to finish the project. Therefore, the critical path has the least slack or float time of any path through the network diagram. Ideally, none of the activities in the critical path should have slack or float time, because any delay in the critical path delays project completion.
Four Activity Variables
Each activity in a project can be defined using four variables, known as early start, or ES, early finish, or EF, late start, or LS, and late finish, or LF. These variables simply represent the earliest and latest times that an activity can start and finish.
Slack or float time for an activity is the difference between its early start and early finish, or the difference between its late start and late finish. You can define lack or float time by the formula Float = LS - ES, or Float = LF - EF.
Types of Float
The term free slack, or free float, describes the length of time by which an activity can be delayed without delaying the early start of any subsequent activity, or activities. The term total slack, or total float, on the other hand, describes the length of time it can be delayed, beyond it's early start, without delaying the finish date for the whole project.
The term independent slack, or independent float, describes the length of time by which an activity can be delayed if all previous activities start as late as possible and all subsequent activities start as early as possible. Independent float is associated with just one activity, rather than two or more.