Stocks, flows, CLDs, and +/-, Ss/Os, and u-shaped arrows ,

[George Richardson]

On Mon, 2 Jun 1997, Phil Odence wrote:
[...]
> Instead of using +/- or s/o, we use a little u-turn symbol (a U with an
> arrowhead on one end) to indicate inverse relationships. Links without
> symbols are by default sames.

The little u-shaped arrows have the same problem in causal-loop
diagrams that Ss and Os have, which was outline in a recent issue of the
Systems Thinker and will be appearing in the System Dynamics Review
shortly.

Namely -- one can not accurately label with Ss and Os or u-shaped arrow
symbols any links in a CLD that represent rate-to-level (flow-stock)
connections. The link from deaths to population must be labeled -, since
the link substracts. It cant be labeled with an O or a little u-shaped
arrow because the rate and the level do not necessarily move in opposite
directions. The level always declines from the effect of deaths, whether
deaths are increasing or decreasing.

...GPR

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George P. Richardson G.P.Richardson@Albany.edu
Rockefeller College of Public Affairs and Policy Phone: 518-442-3859
University at Albany - SUNY, Albany, NY 12222 Fax: 518-442-3398
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[Phil Odence]

>The little u-shaped arrows have the same problem in causal-loop
>diagrams that Ss and Os have

I quite agree with George, but I dont think the "-" is the answer. The
inherent limitation of CLDs George described (i.e. no Stock/Flow
distinction) is exactly why we dont use em too much.

***************************************************************************

Phil Odence
podence@hps-inc.com
High Performance Systems
45 Lyme Road, Suite 200
Hanover, NH 03755-1221
voice- 603 643 9636 x107, fx- 603 643 9502, web- http://www.hps-inc.com