Stocks (also known as levels, accumulations, or state variables) change their value continuously over time. Flows, also known as rates, change the value of stocks. In turn, stocks in a system determine the values of flows. Intermediate concepts are known as dynamic variables and can change instantaneously.
When constructing a stock and flow diagram, consider what variables accumulate over a period of time. In our model, adopter and potential adopter populations are stocks and the adoption rate – a flow.
The system dynamics presentation of the model is shown in the following figure. Stocks are denoted with squares, flow with a valve, and dynamic variables with circles. Arrows denote causal dependencies in the model.
