That is, problems are modeled, not systems. It is not unusual, therefore, for system dynamics models to embody knowledge from both the natural and social sciences. A stock can be thought of as a bathtub. Figure 15.1 below shows some examples of stock and flow structures. In a system dynamics model, stock and flow structures are embedded in feedback loops.
Positive loops generate self-reinforcing behavior and negative loops generate goal seeking behavior.
An example of a positive loop is presented in Figure 15.2. Inspection of the figure reveals that as VHS VCRs become more prevalent, there is more demand for VHS format tapes, which feeds back to generate more demand for VHS VCRs. An example of a negative feedback loop is presented in Figure 15.3. Reference Modes As mentioned above, system dynamics modeling is problem-based. Thus, a system dynamics model cannot be built until a definition of the problem to which it will be addressed is arrived at.