Subject: Systems Language - closed loops

Posted by Warren Farr on 2/5/2010
In Reply To:Systems Language - closed loops Posted by Niall Palfreyman on 2/5/2010

Message:

Niall wrote:
>Here it is no longer a
>question of two separate agents influencing each other, but rather a
>process influences a stock, which in return dictates (not influences)
>the process. There is something about this SD way of viewing the loop
>which my students find difficult to absorb, and it seems to them a
>needless abstraction.

This is closely related to a previous comment about representing stocks that can not go negative and preventing a stock overflow condition.
This type of "direct" feedback where the stock influences its own flow (either inflow or outflow) can be referred to as a "first order control". This simply means that the flow's dependence on the stock value prevents the stock's value from going outside its limits.

The easiest example is a death rate outflow, which is set up to be a fraction of the population. If the population shrinks, the death rate will also shrink since it is some fraction (fixed or variable) of the population itself. If the population is zero, by definition so too is the death rate (preventing a negative population value).

My hope is that this additional concept (to the discussion) may create some additional intuition around the fundamental importance of this type of "small loop" system.

Warren Farr
WPI SD Masters Candidate