Definition
In System Dynamics, a Stock (or Level) is an accumulation point that represents the state of a system at any given time. Mathematically, a stock is the integral of its inflows minus its outflows over time:
- How to read: “The stock at time t equals the initial stock at time t zero plus the integral from t zero to t of the net flow (inflow minus outflow) with respect to time.”
- Meaning: The current level of a resource is the sum of everything that has ever flowed in, minus everything that has ever flowed out, starting from an initial condition.
Why It Matters
Stocks are the source of systemic inertia and memory. They explain why systems do not change instantaneously; for example, even if you stop all new CO2 emissions (flows), the atmospheric concentration (stock) remains high for centuries.
Core Concepts
- Accumulation: Stocks represent the “inventory” of a system (e.g., water in a bathtub, money in a bank, population in a city).
- State Representation: The values of all stocks in a system provide a complete description of the system’s state at a specific moment.
- Independence of Time: While flows are measured over an interval (e.g., dollars per year), a stock is measured at a specific point in time (e.g., total dollars now).
- Conservation: Stocks can only be changed by flows. You cannot change a stock directly; you must change the rates flowing into or out of it.