detachable, to permit adjustments in the plan before
Because the system splits the project into
individual events, estimates and lead times are more
accurate. Deviations from the schedule are quickly
Manpower, material, and equipment
resources are easily identifiable. Since the network
remains constant throughout its duration, it is also a
statement of logic and policy. Modifications of the
policy are allowed, and the impact on events is
Identification of the critical path is useful when
you have to advance the completion date. Attention
can then be concentrated toward speeding up those
relatively few critical events. The network allows
you to accurately analyze critical events and provides
an effective basis for the preparation of charts. This
results in better control of the entire project.
The main disadvantage of network analysis as a
planning tool is that it is a tedious and exacting task
when attempted manually. Depending upon what the
project manager wants as output, the number of
activities that can be handled without a computer
varies but is never high.
Calculations are in terms of the sequence of
activities. Now, a project involving several hundred
activities may be attempted manually. However, the
chance for error is high. Suppose the jobs are to be
sorted by rating, so jobs undertaken by Utilitiesmen
are together as are those for Equipment Operators or
Construction Electricians. The time required for
manual operation would become costly.
On the other hand, standard computer programs
for network analysis can handle project plans of 5,000
activities or more and can produce output in various
forms. However, a computer assists only with the
calculations and print plans of operations sorted into
The project manager, not the
computer, is responsible for planning and must make
decisions based on information supplied by the
computer. Also, computer output is only as accurate
as its input, supplied by people. The phrase garbage
in, garbage out applies.
A network represents any sequencing of priorities
among the activities that form a project. This
sequencing is determined by hard or soft
dependencies. Hard dependencies are based upon the
physical characteristics of the job, such as the
necessity for placing a foundation before building the
walls. A hard dependency is normally inflexible.
Soft dependencies are based upon practical
considerations of policy and may be changed if
circumstances demand. The decision to start at the
north end of a building rather than at the south end is
Network procedures are based upon a system that
identifies and schedules key events into
precedence-related patterns. Since the events are
interdependent, proper arrangement helps in
monitoring the independent activities and in
evaluating project progress. The basic concept is
known as the critical path method (CPM). Because
the CPM places great emphasis upon task
accomplishment, a means of activity identification
must be established to track the progress of an
activity. The method currently in use is the
activity-on-node precedence diagraming method
(PDM), where a node is simply the graphic
representation of an activity. An example of this is
shown in figure 9-8.
Precedence diagraming does not require the use
of dummy activities. It is also easier to draw, and has
greater applications and advantages when networks
are put in the computer. In precedence diagrams, the
activity is on the node.
Activities and Events
To build a flexible CPM network, the manager
needs a reliable means of obtaining project data to be
represented by a node. An activity in a precedence
diagram is represented by a rectangular box and
identified by an activity number.
The left side of the activity box represents the
start of the activity.
The right side represents the
Figure 9-8.Precedence diagram.