Evaluation method

System Dynamics

System Dynamics is a simulation approach for strategy and policy design devised by Professor J.W. Forrester of the Massachusetts Institute of Technology in the late 1950s. In System Dynamics, changes in a social system are modelled qualitatively by causal loop diagrams (Figure 1) and quantitatively by stock and flow diagrams (Figure 2). Non-linear changes, feedbacks, and temporal delays can be incorporated into simulation models with appropriate stocks and flows. The System Dynamics approach has been widely applied to simulation analyses in the field of public health with dedicated software.

Figure 1. An example of a causal loop diagram.

The reinforcing loop shows that increased births lead to increased population, and an increase the population leads to further increased births. The balancing loop shows that increased deaths lead to decreased population, and anincrease in the population in turn lead to increased deaths.

Figure 2. An example of a stock and flow diagram.

Population is a stock variable that can be increased or decreased over time. The stock of population is accumulated or depleted by births (inflows) and deaths (outflows). The birth rate and the death rate are constant.