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Unformatted text preview: now how to do them manually. My belief is that unless you know how the computations are done, you do
not fully understand the meanings of float, early and late dates, and so on. Further, you can easily fall prey to
the garbage-in, garbage-out malady. So here is a brief treatment of how the calculations are done by the
computer. (For most schedules, the computer has the added bonus of converting times to calendar dates,
which is no easy task to do manually.) First, consider what we want to know about the project. If it starts at
some time = zero, we want to know how soon it can be finished. Naturally, in most actual work projects, we
have been told when we must be finished; that is, the end date is dictated. Further, the start date for the job is
often constrained for some reason: resources won’t be available, specs won’t be written, or another project
won’t be finished until that time. So scheduling usually means trying to fit the work between two fixed points
in time. Whatever the case, we want to know how long the project will take to complete; if it won’t fit into the
required time frame, then we will have to do something to shorten the critical path.
In the simplest form, computations are made for the network on the assumption that activity durations are
exactly as specified. However, activity durations are a function of the level of resources applied to the work,
and if that level is not actually available when it comes time to do the work, then the scheduled dates for the
task cannot be met. It is for this reason that network computations must ultimately be made with resource
limitations in mind. Another way to say this is that resource allocation is necessary to determine what kind of
schedule is actually achievable. Failure to consider resources almost always leads to a schedule that cannot be
www.erpvn.net Failure to consider resource allocation in scheduling almost always leads to a schedule that cannot be
The first step in network computations is to determine where the critical path is in the schedule and what kind
of latitude is available for noncritical work under ideal conditions. The ideal situation is one in which
unlimited resources are available, so the first computations made for the network are done ignoring resource
requirements. This is the method that will be described in this chapter; for information on resource allocation
methods, the reader is referred to scheduling software manuals. NETWORK RULES
Two rules are applied to all networks in order to compute network start and finish times. (Other rules are
sometimes applied by the scheduling software itself. These are strictly a function of the software and are not
applied to all networks.)
Rule 1: Before a task can begin, all tasks preceding it must be completed.
Rule 2: Arrows denote logical precedence. The length of the arrow or its angular direction have no
significance. (It is not a vector but a scalar.)
Initial schedule computations are made on the...
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This document was uploaded on 09/27/2013.
- Fall '13