CHAPTER 8
Symbols for Flow Diagrams

The model structure of Chapter 6 and the interrelationships of the equations of Chapter 7 can be displayed diagrammatically. Such a flow diagram helps to prevent confusion and forms another means of communicating the nature of the model to those who are not proficient in "reading" mathematical equations. This chapter explains the symbols that will be used in Chapters 15 to 18.

EXPERIENCE from teaching the formulation of industrial dynamics models has demonstrated that a pictorial representation of an equation system is highly desirable. A diagram that displays the interrelationships between equations helps to lend clarity to the system formulation. Many people visualize the interrelationships better when these are shown in a flow diagram than they do from a mere listing of equations. A detailed flow diagram supplements a set of equations. It gives much of the same information but in a different manner. A properly constructed flow diagram is better than a set of equations for communicating the structure of a system to many practicing managers. A diagram represents an intermediate transition between a verbal description and a set of equations.

A flow diagram of the system interrelationships should be developed simultaneously with the development of the equations of a system. Many beginners attempt to develop equations first and then summarize these later in a diagram, if the diagram is used at all. Such neglect of a diagram until a later stage loses the clarifying advantage that a diagram can provide in the initial stages of equation formulation.

This chapter will describe a set of standard symbols for flow diagrams of dynamic models.1 Symbolism for diagrammatic presentation is based on arbitrary choices that are selected to emphasize and clarify particular aspects of a situation.

The system of symbols used here shows the existence of the interrelationships in the system. It distinguishes levels from rates. It separates the six flow systems of information, materials, orders, money, personnel, and equipment from one another. It discloses what factors enter into each decision (rate) function. But the diagram does not reveal what the functional relationships are within the decision functions. For the specific nature of the interactions between the factors entering into a decision, the diagram carries the equation numbers, and we are thereby referred to the pertinent equation. The diagram corresponds point by point to the equations.


8.1 Levels

A level is shown by a rectangle, as in Figure 8-1. In the upper-left corner is the symbol group (IAR) that denotes this particular level variable.

____________________
1

In Chapter 15 these symbols will be applied to a specific situation.

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