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A system dynamicist studies feedback and time evolution using system dynamics models. Stocks describe accumulated quantities and flows describe their rates of change, while feedback links the rates to the current system state.

For a simple stock $N$ with inflow $a$ and proportional outflow $bN$, the model is $\dot{N}=a-bN$. Its equilibrium and adjustment timescale follow from the equation. A useful model also states the evidence and assumptions behind the chosen feedback relationships.