First of all, float and slack are two words that mean the same thing. It is perfectly fine to use either term in project management. Float is a measure of flexibility in the project schedule. There are two kinds of float, total float and free float. Total float is usually called float. (Sometimes it seems that we try to make things unnecessarily confusing.) Total float is the amount of time that an activity can be delayed without having to reschedule the project completion date. Free float is the amount of time an activity can be delayed without having to reschedule any other activity in the project.
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Total float is a measure of flexibility in the project schedule. It tells the project manager how many days an activity can be delayed before the project completion date must be delayed. This is valuable information. If there is trouble in the project, the project manager may divert resources from one activity to another. The obvious places to divert resources from are the activities that have considerably large amounts of float.
If, for example, an activity having fifteen days of total float has not yet started and the resources are needed to solve a problem in another area for five days, the resources can be diverted to the problem, and the start of the activity with fifteen days of total float can be delayed five days. By doing this the float will be reduced from fifteen days to ten days.
The calculation of total float is very simple. In the calculation of the schedule we calculated the early start, early finish, late start, and late finish dates. To calculate total float simply subtract the early start from the late start. You could also subtract the early finish from the late finish since both sets of dates are separated only by the duration of the activity.
The problem with using total float is that delaying an activity with total float only tells us that we will not have to reschedule the finish date of the project. It does not tell us how many other activities will have to be rescheduled in the project. To help us with this we need to know about free float.
Free float is the amount of time that an activity can be delayed without having to reschedule any other activity in the project. Free float is determined by comparing the early finish—if the relationship between the activities is a finish-start relationship—of the predecessor activity with the early start of the successor activity. (These are sometimes also called the independent activity and the dependent activity.) If there is a difference, it means that the predecessor activity can be rescheduled without having to reschedule the successor activity. If another kind of relationship exists between the two activities then the early start or finishes of each may need to be checked instead.
In NETWORK DIAGRAM WITH FLOAT, activities 7 and 12 are the only activities with free float. Activity 12 has free float because it has no successor and must finish only before or at the end of the project, March 16, as determined by activity 11.
Activity 7 can be delayed up to two days without affecting its successor, activity 10. All the other activities that have float must reschedule at least one other activity in order to allow the delay and use the float.
One thing that should be noticed about total float is that when it is used up in one activity by a delay, the successor activities will lose some of their float as they are rescheduled. Notice activities 3, 5, 9, and 12. If activity 3 is delayed two days, activities 5, 9, and 12 must have their early start and early finish dates rescheduled as well. This in turn reduces the total float of each by two days as well. Notice also that delaying an activity within its total float does not affect the late schedule dates; they remain the same.
The use of free float does not affect the other activities; the early start and early finish dates of the activity with free float are the only ones affected if we reschedule the activity within the amount of the free float. This means that an activity with five days of free float could be rescheduled to start or finish five days later without having to reschedule the project completion date or any other activity in the schedule.