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108 PRACTICAL SCHEDULING

• Precedence-driven The work is scheduled by the computer, based on task
durations, constraints, and relationships that have been defined. A pure
precedence-driven schedule may not fully support the defined milestones,
and assumes that all resources will be available as needed. While this cer-
tainly is not realistic, it’s a good place to start. Even a schedule that consid-
ers the milestones and the resources must also consider precedence
relationships if it is to have any validity.

• Resource-driven The work is scheduled when the resources are available
to do the work. To do this, we need to start with a preliminary (not resource-
constrained) schedule, preferably one that is precedence-driven. Then we
define the resources that are to be assigned to the work, and let the com-
puter compute the required resource loads. By also defining the available
resources, the computer can compare resource requirements to resource
availability. Then by invoking the resource leveling function, the computer
can reschedule the work to stay within defined limits. We get into this in de-
tail in Section 4.

A practical final schedule will be one that considers all the above. In doing so,
there will be contention for scarce resources, conflicts with established mile-
stones, haggling over priorities, political and territorial squabbles, and considera-
tion of risk. Task durations will be challenged, defined task precedence will be
redefined, and even the defined workscope may be modified. Resource availabil-
ity will also be extremely dynamic, changing almost as fast as it is defined.

Obviously, the computer becomes an essential tool to deal with project sched-
uling. In this chapter, we provide some tips on how to use these tools to address
all of these scheduling dynamics to effectively build a practical project schedule.

Schedule Analysis Using Total and Free Float

The use of float, for schedule decision making, goes back to the original PERT
and CPM programs of the late 1950s. It is still a valuable technique, if used
properly, and not blindly. Float (also called slack in Microsoft Project) is calcu-
lated by the critical path scheduling function that is the core of virtually all
project management software products. Float represents the difference be-
tween the earliest time that a task can be performed and the latest allowable
time. There are two types of float: total float and free float. Each type can be
used differently.

Total float is the duration that a task can slip without extending the end date of
the project. The more total float, the more time contingency there is in the proj-
ect. We can use this information for two key purposes. The first is to determine