CrbnBlu@aol.com (aka Gene Bellinger) wrote:
snip
> As I started using Vensim PLE if was pleased that I could build the ST
> diagrams in the same software package I would build the model in. After
> building the first couple ST diagrams and then destroying them in the process
> of converting the ST diagram to a Stock-Flow diagram 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. It took a couple
> days for this to really sink in, and then about two hours to reorient my mind
> as to the implications of this.
Gene: Youve stepped into the middle of what seems (IMHO) to be an
almost-religious controversy. One school of thought extolls the virtues of the
causal loop diagram as the entre to thinking systemically about a problem. The
other decries the use of the CLD until AFTER you have built the stock and flow
model ... then the CLD becomes a useful way to summarize the system in question.
Personally I find myself squarely in both camps ... the argument almost sounds
like a MAC vs PC or WordPerfect vs Word argument (or Wordstar vs EMACS, depending
on where in the baby-boom you find yourself) -- whatever you grew up on is the
best way to look at the world. In some ways, the discussion about the merits of
Vensim vs IThink vs PowerSim is right here. I grew up in a STELLA/IThink
environment, so the S/F approach is the best FOR ME IN MOST CIRCUMSTANCES. But
this seems more than just a matter of preference ... Barry Richmond, for example,
in a very insightful paper on the HPS web site, suggests that using the CLD as an
exploration tool actually works AGAINST developing a profound understanding of the
system in question, and feels that the use of the archetype by those not already
versed in ST/SD is dangerous and probably counterproductive.
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 didnt start with the
CLD, for reasons related to the science. On the other hand, Senge and most of the
folks at Innovation Associates (and MIT for that matter) see power in the CLD as
both simple, and explanatory --- a good introduction to systems thinking concepts.
I have argued against the casual use of the CLD because it is inherently
undisciplined (as you suggest in your last paragraph) ... it it too easy to create
stocks that seem to be affected by stocks ... a no no in Forresters eyes.
I take issue with Bob Eberleins 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. Thats just too much for me.
Personally, I find the CLD to be easier for people to grasp, but also much less
effective in promoting structural learning. Stocks and flows contain a much
needed discipline --- what are nouns, what are verbs, what things are conserved,
what things are transformed, etc. Once people get really good at it, anything
will do. The question in my mind becomes "what works best given my current
objective." If it looks like we will be going to modeling, Id rather start with
stocks and flows. If it seems as if we will be able to gain the necessary insight
without resorting to modeling, the CLD is easier. Trick is to know in advance, I
guess.
> The end result being that I am now quite comfortable modeling and simulating
> ST diagrams, and I dont even miss the Stock-Flow diagrams. Now I no longer
> need two different pictures.
Thats cause you already understand what youre doing. A court case in England
put the game of darts on trial ... if it was a game of chance, gambling about the
outcomes would be illegal ... if it was a game of skill, gambling would be ok.
The counsel called one witness, who picked up three twenty-penny nails and threw
three bulls. Luck or skill? Moral is, when youre good at it, you can use any
tool to get the end you want. Its novices who buy the $500 titanium adjustable
weight and balance darts.
> Your first thought about this is probably related to how one tells the
> difference between Stocks, Flows, Converters, and Information Flows. I have
> found that color coding the components of the model works just fine.
This is how YOU impose the discipline of S/F diagrams to the CLD. I rest my case.
Hope this didnt sound too pedagogical ... but its an area Im very concerned
about. You only get to introduce systems thinking once in an organization
(innocculation phenomenon), so you have to choose the right track ... thats where
I am right now.
Steve
--
Stephen B. Wehrenberg, Ph.D.
Chief, Forecasts and Systems, US Coast Guard;
Administrative Sciences Program, The George Washington University;
wstephen@erols.com