Network Planning and Control

a planning and control technique based on the use of network diagrams. It finds application in the planning and management of, for example, the development of large industrial complexes, scientific research, the design and setup work required to bring new products into production, construction and renovation projects, and major repair work on fixed capital stock. The network technique permits establishment of the interrelationship between planned activities and obtained results, makes possible more precise calculation of the plan, and allows timely correction of the plan. The use of electronic computers in the management and construction of automatic control systems is based on the network technique (seeAUTOMATION OF PRODUCTION).

Figure 1

The essence of the network technique consists in the construction of a logicomathematical model of the controlled system in the form of a network, or arrow, diagram (see Figure 1). Alternatively, the model can be in the memory of an electronic computer. The model reflects the interrelationship and duration of a certain set of activities (seeMATHEMATICAL MODEL). After the network diagram is optimized by the methods of applied mathematics and computer engineering, it is used to control the activities.

The network comprises activities and events. Each event represents the completion or the beginning of an activity. An activity is the action that must be taken in order to progress from one event to the next. In the diagram, events are symbolized by circles, and activities are shown by arrows indicating the links between events (alternatively, the activities can be represented by circles, and the links between them by arrows). An activity must be specific and clearly defined, and it must be assigned to a responsible manager. The duration of the activity can be measured in such units as days, weeks, or tens of days and is indicated above the arrow in the diagram. The time estimates are supplied by the managers responsible for the activities in question. All activities in the diagram lead to the final event—the planning goal.

Established standards and experimental data are used in planning the duration of the activities. Often, however, the time an activity takes cannot be expressed by a single reliable estimate. This is the case, in particular, when new products are being brought into production or when scientific investigations devoted to solving particular problems are being carried out. The responsible manager then usually supplies three estimates: (1) an optimistic estimate (minimum duration of the activity t_min), which is the minimum time in which the activity can be accomplished under the most favorable conditions; (2) a pessimistic estimate (maximum duration of the activity t_max), which is the time required to complete the activity under the most unfavorable conditions; and (3) the most likely duration of the activity (t_ml), which is the time the activity takes under normal conditions.

The expected time of an activity is determined on the basis of three or two estimates by means of one of the following formulas:

An important part of the development of a network diagram is the determination of the time required for the various paths. In Figure 1 the paths are represented by the lines formed by the arrows of interrelated activities; the ends of the lines indicate the initial and final events. A distinction is made between complete paths and critical paths. A complete path originates at the initial event of the network and ends at the final event of the network. The critical path is the path with the greatest duration; it determines the time required to complete the entire set of activities, or the entire project—that is, the time required to accomplish the final goal. In Figure 1, the critical path is indicated by heavy arrows.

The critical path is the most important path in the network technique, since it is the basis on which an optimal plan is selected and monitoring of the progress of the project is organized. The ratio of the duration of any path to the duration of the critical path indicates the path’s degree of slack. Since the critical path requires the longest time from the initial event to the final event, all other events and activities must be located on shorter paths. In English, the use of the concept of the critical path in planning and control is known as the critical path method (CPM).

Improved forms of the network technique also include data on material expenditures and on cost increases as the project develops. A description of the usual sequence of steps in the network technique follows. The set of activities is broken down into separate successive stages, each of which is assigned to a responsible manager. The events and activities required to accomplish the final goal are identified and described. Then the network diagram is constructed. Estimates are made of the time required to complete each activity in the network. The critical path and slack times are computed. After the network is analyzed and the diagram is optimized, measures are developed to reduce the time of the critical path. Finally, control of the progress of the project is carried out with the aid of the network diagram.

Each manager determines the composition and sequence of the activity stage for which he is responsible. The person responsible for the project then compiles the initial network diagrams. After corrections are made, these diagrams are joined together in a composite network diagram. The final event in this diagram represents the specified final goal. When the composite diagram is constructed, particular attention is paid to the elimination of discrepancies at the points where the initial network diagrams come together—that is, at the points where the individual activity stages meet.

Network plans become less detailed at higher levels of project execution. If they are intended for the managers of enterprises, they contain only the accomplishment dates for boundary events (which are input events for some enterprises and output events for others) along with the times for starting and finishing activities in the critical zone. Network plans for middle-level managers also carry information on the accomplishment dates of the boundary events for the individual responsible managers.

The network model is continually monitored, corrected, and adjusted while the plan is being carried out. To eliminate discrepancies between the planned and the actual progress of the project, organizational and technical measures are taken (seeORGANIZATIONAL-TECHNICAL MEASURES PLAN).

Thus, the network technique permits the activities of the project to be carried out in their logical sequence. Network diagrams facilitate a systems approach to the organization of control of given processes, since the staffs of the various subdivisions take part in the processes as links of a single, integrated organizational system that are united by a common task.