MOD 4.2

• Stocks and flows are the basic building blocks of system dynamics models.
• Jay Forrester originally referred to them as “levels” for stocks and “rates” for flows.
• A stock variable is measured at one specific time and represents a quantity existing at that point in time, which may have accumulated in the past.
• A flow variable is measured over an interval of time.
• A flow would be measured per unit of time, such as a year.
• Stocks are entities that can accumulate or be depleted, such as a bathtub, which fills with water from a faucet.
• Inventory and Installed Base are examples of stocks.
• Flows are entities that make stocks increase or decrease, like a faucet or drain affects the level of water in a bathtub.
• Production, which increases Inventory, and Purchases by Consumers, which increases Installed Base, are examples of flows.
• Flows are the only variables that can change stocks.