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Stock and flow - Definition and Overview |
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Stocks and flows are the basic building blocks of system dynamics models. Jay Forrester originally coined them as "levels" (for stocks) and "rates" (for flows).
A stock (or "level variable") is some entity that is accumulated over time by inflows and/or depleted by outflows. Stocks can only be changed via flows. Mathematically a stock can be seen as an accumulation or integration of flows over time - with outflows subtracting from the stock. Stocks typically have a certain value at each moment of time - e.g. the number of population at a certain moment.
A flow (or "rate") changes a stock over time. Usually we can clearly distinguish inflows (adding to the stock) and outflows (subtracting from the stock). Flows typically are measured over a certain interval of time - eg. the number of births over a day or month.
Examples
| Stock | Inflow(s) | Outflow(s)
| | inventory | incoming goods | outgoing goods
| | guests in a hotel | guests arriving | guests leaving
| | population | births
immigration | deaths
emigration
| | water in bathtub | water pouring in | water leaving through sink
| | waste in disposal site | dumping waste | decay of waste
| | bank account | paying in | withdrawals
| | fuel tank | refueling | fuel consumption
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Example Usage of Stock |
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EdSeveralNHS: If u live in the NE, are u going to get snow removal stuff? Do u have it in-Stock if u r a retailer? #EdSeveralNHS |
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86thTry: Thinking about selling some of my thousands of pictures on a Stock site. But which one... |
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KT_Riffic: Thinking about selling some of my thousands of pictures on a Stock site. But which one... |
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