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CPM BASICS 79

tion of one is dependent upon the completion of another. Here, you would use
the Finish to Finish (FF) relationship. For instance, task B can finish 5 days after
the completion of task A (FF-5). The duration of the delay (in this case, 5 days) is
called the lag.

Tip A practical way of working with critical path scheduling
is to start off by defining most relationships as default FS de-
pendencies. Then, after the project schedule has been calcu-
lated, use the ability to overlap tasks to selectively compact
the project duration. See Chapter 3.4 for discussion on the
value of time compression and Chapter 3.5 for more tips on
practical scheduling.

CPM: How It Works There are several options for imposing date constraints on
the schedule. In order to understand how these work, we have to pause a moment
to review how critical path scheduling works.

After the project model has been defined to the computer (task definition, task
durations, dependencies, and imposed date constraints), the computer makes a
forward pass to determine the earliest start and finish of all tasks. Then, after de-
termining the earliest project completion date, the computer makes a backward
pass from that end date, to calculate the latest start and finish date of all tasks that
would support the project end date.

The difference between the early dates and the late dates is called total float or
total slack. The path through the critical path network that has the least amount
of float or slack is called the critical path. If the user has not imposed a required
end date on the project, the critical path would have a float/slack of zero. If a proj-
ect end date is imposed that is earlier than the freely computed end date, there
will be tasks that have negative float. The tasks having the largest amount of nega-
tive float are said to lie on the critical path.

Therefore, it can be noted that any tasks that lie on the critical path cannot be
delayed without delaying the completion of the project.

Tip The terms float and slack are used interchangeably. They
are the same. Float had been the common usage, until Mi-
crosoft introduced their scheduling products, which substi-
tuted slack for float.