cf: /home/chiao/Fuzzy/Literature/SystemDynamics/15-988-fall-1998-spring-1999/contents/readings/teachers.pdf
The level is an accumulation, or integration, or stock, to choose terminology from different fields. The rate is a flow that changes the amount in the level. The rate is defined by a “policy” statement that tells how the rate is controlled by the value of the level in comparison to a goal. All systems, everywhere, consist of these two kinds of concepts --levels and rates--and none other.
Water in a bathtub is a level, the flow of water is the rate that changes the level. A person’s reputation is a level that is changed by the flow of good and bad actions by that person. The degree of frustration in a group is a level that gradually changes in response to surrounding pressures.
At the point where a rate of flow is being determined, the condition of the system, as indicated by the values of the levels, is compared with a goal to determine control of the flow.
A stock is the term for any entity that accumulates or depletes over time. A flow is the rate of change in a stock.
