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CPM BASICS 81

First, it prevents determination of a schedule that is based on
defined dependencies. Second, it makes updating the sched-
ule much more difficult, as the user must go in and manually
change all of the imposed dates.

Often, this overuse of imposed Start On dates is motivated
by a desire to avoid the effort of defining all the dependen-
cies. Ironically, the result is not only a poorly developed sched-
ule, but also vastly increased effort to maintain it.

Furthermore, one of the great benefits of project manage-
ment software comes from using these tools to help develop a
supportable schedule, based on defined work, dependencies,
and available resources. Ignoring all of this to create a forced
schedule may be easier to do and more politically acceptable.
But if it is not supported by the facts, what good is it in the
long run?

The Finish No Later Than (FNLT) constraint works in just the opposite man-
ner from the SNET constraint. The FNLT date, when imposed, affects the calcu-
lation of the late dates. This can be best illustrated by an example. Let’s say that
the calculated schedule for construction of a house says that the roof can be com-
pleted as late as March 15 in order to meet the contract house completion date of
June 30. We are in Iowa and the snow can be expected to make a substantial ap-
pearance by December 1. It is decided that it is important to have the roof up by
that date in order to be able to work inside the house and to protect the materials.
By imposing a FNLT date of 12/1 on the roof completion task, we then drive all
other late dates to support that imposed constraint. The FNTL date does not
have any effect on the early dates, which are computed during the forward pass.

Other Constraints Some of the constraints on scheduling the work of the proj-
ect may involve events and conditions that are not on the list of defined tasks. We
may be waiting for the availability of the plot on which to erect the house. Or we
need the building permit. Erection of the roof trusses may require the availability
of a crane. Purchasing of the erection materials may be dependent upon the avail-
ability of funds. Placement of the windows may be constrained by completion and
approval of the final construction drawings. Availability of the electricians may
depend on completion of the electrical work on another building.

Actually, all of these constraints should be incorporated into the project
schedule. For most of those in the preceding list, we would create a new task
(which may have a zero duration) to note the constraint. We would enter a