System Dynamics

System Dynamics Models and Representations

The fundamental model used in system dynamics is called a stock and flow model. However, in order to create such a model, system dynamicists typically engage in a knowledge elicitation process and create an initial causal loop diagram (aka a causal influence diagram). Both can be useful in supporting learning and instruction, as argued by Marcelo Milrad and colleagues, and the stock and flow model can serve as the basis of an interactive simulation when the components are mathematically specified, according to John Sterman. A causal loop diagram is intended to represent all of the major factors and their relationships that are involved in a complex and dynamic system. While its primary use is in eliciting information from experts about the system, it is also used to convey information to others about how a complex system operates. In addition, the process of developing a causal loop diagram has been shown to be an effective indicator of how well someone understands the complexities of a problematic situation and, as a consequence, can be used for assessment and feedback to support the learning process. Figure 1 is a simplified causal loop diagram representing a population model. Note that two death rates are included—one for adults and one for children. The person developing the diagram could have included additional death rates (e.g., to explicitly represent infant mortality rates or death rates for adolescents). The point here is twofold: (1) first, there is typically not a single correct representation for a complex and dynamic situation, and (2) the representation should include components that might be targets for human intervention. With regard to the latter, it could easily be imagined that proper prenatal care might impact infant mortality rates; therefore, it could be argued that this relationship should be made explicit in the figure. The purpose and projected use of the representation is an important consideration. A general advantage of a causal loop diagram is that it supports a holistic view of a complex and dynamic system in a single figure represented on a single page or screen. Such representations address a common deficiency in human reasoning—namely, the tendency to ignore significant portions of a complex system, as shown by Dietrich Dörner as well as by Peter Senge. As Figure 1 indicates, delayed effects can also be represented, which is an additional challenge; all too often, humans expect to see nearly instantaneous effects of a decision or action, but real systems often involve significant delays. Positive and negative feedback loops are depicted through the use of influence indicators. In Figure 1, a plus sign indicates a change in the same direction between the connected nodes, while a minus sign indicates a change in the opposite direction between the connected nodes. In general, a loop with an odd number of minus signs is a negative feedback loop (aka a balancing loop), while a loop with an even number of minus signs is a positive feedback loop (aka an escalating loop). The loop connecting children and maturing adolescents is a balancing loop as there is one minus sign in that loop.

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