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CHAPTER 8
PURCHASING ANALYSIS TOOLS AND TECHNIQUES
8.6.4 THE IMPORTANCE OF SCHEDULING
The primary reason for scheduling a project is to ensure that the deadline can
be met. The critical path method helps to identify the activities that will determine
the end date and it provides focus on the key tasks that must be managed
to help ensure that the project is completed on time. It is very easy to create
schedules that look good on paper but won’t work in practice. One of the main
reasons this occurs is a lack of resources with which to do the work when it
comes due. In fact, unless resource allocation is handled properly, schedules
are next to useless. Fortunately, there is scheduling software to handle resource
allocation well.
8.6.5 CONSTRUCTING AN ARROW DIAGRAM
When constructing a schedule, the general rule is never to plan (or schedule)
in more detail than can be reasonably managed. Some projects, such as
overhauling a large power generator, are scheduled in increments of hours.
Others are scheduled in days, while some big construction jobs are scheduled
to the nearest month.
While planning in too much detail is undesirable, one should also not plan in
too little detail, for example, creating schedules showing tasks having durations
of several months. A good rule of thumb is that no task should last much more
than four to six weeks. A task with an overall duration of several months can be
broken down into sub-tasks comprised of four to six weeks in duration.
There are two ways to develop a schedule. One is to begin at the end and work
back to the beginning. The second method is to start at the beginning and work
towards the end. The most common approach is to start at the beginning. The
first step is to decide what can be done first. Sometimes several tasks can
start at the same time. In that case, simply draw them side-by-side and start
working from there.
The boxes, in the progression in the diagram in Figure 8.5, are numbered
according to the steps taken to place them. In other words, all boxes with a 1
beside them were placed in the diagram in Step 1, and so on. The next steps
are noted as 2 and so on. The duration is in minutes and is noted as DU in the
diagram. Note that it sometimes takes several iterations before the sequencing
can be completed [4].
This small project might be thought of as having three phases: preparation,
execution and cleanup. There are three preparation tasks: pick up rubbish,
get vacuum cleaner and get out window cleaner. The cleanup tasks include
put away vacuum cleaner, put away window cleaner and dispose of rubbish.
This schedule diagram follows a basic rule of scheduling, i.e., to diagram what
is logical and then to look at resource constraints. For a room cleaning project
carried out by one person there can be no parallel paths. On the other hand,