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82 CRITICAL PATH SCHEDULING

SNET date on these “dummy” tasks and make them predecessors to the tasks
that cannot be started until this constraint is satisfied. For instance, you may be
able to survey and lay out the lot while waiting for the building permit. But the
first excavation task would be constrained by a starter task called Obtain Build-
ing Permit. The duration would be zero. A SNET date would be imposed based
on the expected permit date. This task would be defined as a Finish to Start
predecessor to the excavation task. A zero-time task is often called an event, be-
cause it is a point in time.

Tip Creating dummy tasks to note any of the above con-
straints is favorable over just imposing a SNET date on the af-
fected task. There are two reasons for this. First, by using a
distinct task, there is a specific, separate line item in which we
can define the specific constraint. Second, there could be more
than one such constraint for a task. We can use a separate con-
straint task for each.

Still More Constraints Up to this point, we have allowed the schedule to be
computed without considering the availability of labor resources. We have as-
sumed that whatever resources are needed to support the work, when scheduled,
would be available. Obviously, this is not a good assumption. Eventually, we will
want to build a definition of available resources and to allow the program to con-
sider the availability of resources while making an adjusted computation of the
schedule. This is called Resource Leveling, or Resource-Constrained Scheduling,
and will be discussed in detail in Section 4.

If we have defined the resources needed to perform the tasks, we can have the
CPM program calculate the quantity of each resource required for each date.
These quantities may be number of units of a resource (when using classes of re-
sources, such as electricians) or may be number of hours per time period for
named resources, such as Jack Smith, a systems designer. By executing this calcu-
lation of required resource quantities, we can see and evaluate the level of re-
sources needed to support the schedule (prior to adjusting for resource limits).
This process, of computing the required resources against time, is sometimes
called Resource Aggregation.

Where the resource aggregation indicates that there is a significant period of
time where resource needs will peak, you can use this as an early warning to ei-
ther try to arrange for added resources, or to be prepared for significant schedule
adjustments. More on this in Section 4.