Arrow Diagrams Vs Precedence Drawings Vs Timescaled Diagrams

Figure 3-3 shows three methods of drawing CPM diagrams. Each has its pros and cons.

Arrow Diagramming, at present, seems to be the most popular method. This probably stems from the fact that it was the first method to be developed and computerized. It is also easier to associate with time and flow of job activities.

A major difficulty to arrow diagramming is the "dummy" activity. Learning the significance and proper usage of "dummies" requires time and experience. The arrow diagram is also cumbersome to modify.

The second method is Precedence Diagramming. As shown, the activities are on nodes. Length and direction of the arrows have no significance as they indicate only the dependency of one activity on another. This method is commonly referred to as "Activity-on Node."

This method has received wider acceptance over recent years. Its primary advantage is that it eliminates "dummy" activities. It is also easy to modify. Since there are no events in the "Activity-on-Node" diagrams, it is difficult to use milestones in the network; therefore, visual aspects of precedence networks are poor. As there is no dateline, it is also very difficult to view overall status.

Both methods are acceptable, however, arrow diagrams continue to have the slight edge because of early acceptance and familiarity.

The third method, showing a time-scaled network, is just a more "visual" tool of the arrow diagram. It is not designed as a tool for detailed control, but a technique to present overall schedules to management. It gives a quick and simple picture of the schedule as it relates to time, activity interfaces and criticality.

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