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Preserving Float


Float refers to the amount by which an activity's finish can be late while not affecting the rest of the project. There are two kinds of float. Free float refers to the amount by which the activity's finish can be delayed without requiring any other activities to change. Total float is the amount by which an activity's finish can be delayed while still meeting a deadline. For example, consider the optimal schedule for the lawn project example from the overview:



If the deadline for this project is 4:00 pm then the Cut Lawn activity has no float of any kind; if it doesn't finish on time, we'd miss the deadline. On the other hand, the Tidy Path activity has 3 hours of free float and 3 hours of total float; if it finished 3 hours later than scheduled, no other activity would be affected and we wouldn't miss the deadline. The Edge Path activity has no free float; if it doesn't finish on time, the Tidy Path task can't start on time. However, the Edge Path activity has 3 hours of total float; if it finishes 3 hours late, the Tidy Path activity would start late but we could still finish before the deadline.

When calculating total float all deadlines that affect an activity are considered. This includes the project deadline, any finish on or before constraints and any finish on or before or mandatory finish dates on successor activities.

PSOP's Primavera Compatability tab allows you to specify the amount of total float PSOP should preserve. PSOP will ensure that each activity has the requested amount of total float (if possible) even if preserving float results in greater overload. For example, if the lawn project shown above had a deadline of 4:00 pm and you requested that 2 hours of total float the Cut Lawn task would be scheduled to start at 8:00 even though that would cause two hours of overload (the Edge Path and Cut Lawn tasks use the same resource and there is only 1 such resource available).

Of course it is not always possible to preserve the requested amount of float. In the above example if you had requested 10 hours of float there is no way to preserve that amount of float for any of the activities. If the requested amount of float is not available PSOP will preserve as much float as it can. Requesting 10 hours of float on the lawn mowing project would thus result in the early start schedule (every task would be scheduled as early as possible).

Float preservation is relative to the deadline specified in the leveling options. If you had specified a project deadline of January 20 in P3e, but indicated a finish date of January 1 in the PSOP leveling options, PSOP will preserve the requested amount of float relative to the January 1 deadline specified in the PSOP leveling options. If the project is scheduled without a deadline (e.g. no limit is specified as the finish by date) float will be preserved for those activities that have finish on or before constraints or have successors with finish on or before or mandatory finish constraints.