The Critical Path Method (CPM) - Construction Project Scheduling and Control - page 75

Further Discussion of Float
From practical observation, we can deduce the following five points about float:
1. In construction project scheduling, total float (TF) is the most frequently used
type of float. Free float (FF) comes in a distant second. Interfering float and
independent float are almost unheard of, except in some delay-claim resolution
cases. In fact, most commercially available computer software programs do not
calculate these two types of float or even mention them.
2. The project manager should be responsible for distributing the total float and
should understand that total float, in most cases, is shared among several activ-
ities. If the first activity consumes it, the remaining activities will be affected
and may be left without float and hence become critical.
3. Total float may be used for resource leveling, which is explained in Chapter 6.
The project manager may choose a certain day on which to start an activity
(between the early start and the late start) to satisfy certain resource or other
needs. For example, an activity that includes the installation of expensive equip-
ment may have 60 days of float. The owner may elect to delay the installation
as late as possible in order to defer the cost.
4. The question of float ownership is a broad and complicated subject. It is not
only a technical subject but also a legal one. Some contracts spell out whether
the contractor has granted the owner float ownership. If not, the float could
be distributed in different ways. Some regulations give the right of “first come,
first served.” Others suggest distributing it in proportion to the duration,
budget, “priority code,” or other criteria of the activities that share it. There
are many publications on this subject, such as those by Pasiphol and Popescu
(1994) and Ponce de Leon (1986).
5. The project manager may choose to hide or reduce the total float. Scheduling
software provides several tricks for doing so, such as imposing a constraint on
the activity to finish by a certain date. In computer programs such as Oracle
Primavera P6, you can do the following:
• Reduce or eliminate the total float by imposing a finish constraint. For
example, activity H in example 4.3 has an early finish date of day 15 and
a late finish date of day 26, which gives it 11 days of float. If we impose
the constraint Finish No Later Than Day 19, day 19 will replace the late
finish date in the calculation of total float. Thus, TF = 19 − 15 = 4days.
Of course, if we impose the constraint Finish No Later Than Day 15, the
total float will be zero and activity H will be critical. Different software
packages provide different types of constraints: some can override the logic
and some cannot. For example, if we impose the constraint Finish No Later