detachable, to permit adjustments in the plan before its evaluation.

Because the system splits the project into individual events, estimates and lead times are more accurate. Deviations from the schedule are quickly noticed. 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 assessed quickly.

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 various orders. 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.

Elements

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 an example.

PRECEDENCE DIAGRAMING

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.