What are stocks and flows in system dynamics diagrams?
In system dynamics diagrams, stocks (also called accumulations) represent quantities that build up or deplete over time and are depicted as rectangles. Flows represent the rates of change that add to or subtract from these stocks and are depicted as arrows with valves.
Conditions
- The context is introductory system dynamics modeling.
- The model uses standard stock-and-flow diagram notation.
Reasoning, step by step
- Identify the rectangular boxes in the diagram as stocks.
- Recognize that stocks represent accumulations of resources, information, or physical entities.
- Identify the arrows with valves connecting to stocks as flows.
- Understand that inflows increase the stock while outflows decrease it.
- Conclude that stocks change over time based on the net difference between inflows and outflows.
Example
In the deer population model, 'Deer' is a stock (rectangle) representing the total number of deer. 'Births' is an inflow (arrow pointing in) and 'Deaths' is an outflow (arrow pointing out).
Common misconceptions
- Believing that stocks are static values that never change.
- Confusing flows with the actual quantity of the stock itself.
- Thinking that arrows represent physical movement rather than rates of change.
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