Stephen Wehrenberg and Gene Bellinger are having an interesting
discussion regarding system dynamics (SD), stocks and flows (S/F),
and causal loop diagrams (CLD). I'm caught up in a large-scale
modeling effort and feel like chiming in . . .
Gene wrote (snipped):
> I commented to Bob
> Eberline how frustrating it was destroying the initial ST diagram. Bob
> commented that the picture was sort of irrelevant and what really mattered
> was the formulas contained in the elements of the diagram.
>
Stephen responds (snipped):
>I was in Boston last week and addressed this question to just about everyone
>who would stop to listen. Richmond made a strong case for S/F first, CLD last.
>John Sterman is decidedly on the same side. Forrester certainly didn't start
>with the CLD, for reasons related to the science.
I was in Boston, too, and spent Monday in an all-day symposium hosted by
Jim Hines. In attendance were the likes of Eberlein, Sterman, Forrester,
George Richardson, and others well-known to the SD community. The symposium was
intended to bring together practitioners, academics, and clients. Richardson
was clearly an advocate of causal loop diagrams; Jay Forrester was not.
Stephen also writes (snipped):
>I take issue with Bob Eberlein's comment that "... the picture was sort of
>irrelevant and what really mattered was the formulas contained in the
>elements of the diagram." The picture DOES matter if you are trying to help
>others use ST and SD tools to solve problems in situ. A modeler sitting on
>the other side of the transom can use any method he or she is comfortable with ...
>but the objective (and reality) will be a product, not learning. If the objective is
>learning, form takes on great importance. I took this up with him in Boston, and further
>critized what I consider to be the worst of all worlds ... mixing the CLD
>and S/F forms in the same diagram. That's just too much for me.
As a client, I agree. The models have intrinsic value and the modelers don't
seem to grasp that. My explanation is that the models make visible what are
otherwise invisible relationships and literally "seeing" these relationships
is an important aid to understanding the structure and the dynamics of the
system being examined.
Stephen closes with:
>Hope this didn't sound too pedagogical ... but it's an area I'm very
>concerned about. You only get to introduce systems thinking once in an organization
>(innocculation phenomenon), so you have to choose the right track ... that's
>where I am right now.
I share your concern. Right now we're having very good luck with our system
dynamics modeling efforts but I am concerned that SD will become the next
silver bullet. I'm also concerned that we can adequately respond to demand,
which has to do with developing an in-house SD capability.
Fred Nickols
Executive Director
Educational Testing Service
Mail Stop 10-P
Princeton, NJ 08541
fnickols@ets.org
nickols@aol.com
--Learning-org -- An Internet Dialog on Learning Organizations For info: <rkarash@karash.com> -or- <http://world.std.com/~lo/>