|
|
|
|
|
Stock And FlowStocks 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 |
|
 |
|
| Copyright 2005-2009 OnPedia.com. All Rights Reserved |
|
|