[from Gary Cziko 920910.1558]
I just had my first encounter with STELLA (version 2.1) and it sure looks
like an easy way to do PCT-type modelling. Yes, Rick Marken would go
bananas with this program (sorry, I should've said "MORE bananas")!
But I must say that I was distracted by the manual. I didn't expect to get
a tutorial on control theory, but that's part of what's in the first
chapter.
On page 18:
"This ongoing, circular control process in which "CONDITIONS LEAD TO
ACTIONS WHICH RETURN TO INFLUENCE CONDITIONS" is known as a FEEDBACK LOOP.
This closed-loop structure is well-known in engineering, the physical and
life sciences, and in medicine. It is somewhat less-established in the
social sciences, although it is every bit as applicable there. FEEDBACK
CONTROL is an extremely important process; one which is absolutely
essential to maintaining viability in a changing environment. We will
revisit this concept several times throughout the remainder of the Guide."
On page page 37 and 38:
"The "internal feedback structure" viewpoint carries with it two additional
perceptual reorientations which also are alien to many people.
"The first is a shift from a "discrete" to a "continuous view of underlying
processes. Feedback loops, and goal-seeking activities, are CONTINUOUS in
their operation. Thus, for example, you do not continue to ship product
flat out until you have ZERO inventory--at which point you ship zero.
Rather, as inventory falls below DESIRED levels, you would begin to
restrict shipments (allowing your "lead time" to rise). Restrictions would
become increasingly severe as inventory fell increasingly far below desired
levels. A feedback viewpoint thus implies "continuous regulation", not
flip-flop! Your STELLA models should reflect this viewpoint.
"The second reorientation in viewpoint, not unrelated to the first, is a
shift from a static/equilibrium perspective to a dynamic/disequilibrium
view. Much of our thinking is static. For example, it is not uncommon to
hear peopl express sentimentslike: "overwork causes stress". IN such
expressions, causality runs only ONE-WAY: from overwork to stress. But in
fact, it is just as legitimate to say that "stress causes overwork". If
you're stressed, you're [sic] mind's not at ease. Maybe you're not
sleeping too well either. Your diet (and exercise routines) may also be
disrupted. All of these influences comgine to result in a decrease in
yhour work-related productivity. Lower productivity, in turn, means less
work gets done, and more errors get created in the process. Slower work
rates, and more rework , mean more work to do. More work means less sleep,
more pressure, more fatigue, and still lower productivity--thereby
reinitiating the cycle. We've all been there. And, the causality is NOT
linear, it's CIRCULAR!"
The author of the manual and president of High Performance Systems (which
publishes STELLA) is Barry Richard, Assistant Professor at Dartmouth (at
least in 1988 when the manual was published). Does anyone know what his
field is? He certainly would fit into the company on this network. Maybe
we give him a free one-year membership in CSG in return for a steep
discount on STELLA for CSG members!--Gary
P.S. Bill [Powers], I am dreaming of playing with the reorganization model
you showed me in Durango on STELLA. But I still have a lot to learn. I
tried to build a simple control system and got the message "no circular
connections allowed" as I put in the last arrow! I think I forgot to add
an "inflow" source to the model (STELLA makes a distinction between "input"
and "inflow" which is not yet clear to me).
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Gary A. Cziko Telephone: (217) 333-8527
Educational Psychology FAX: (217) 333-5847
University of Illinois E-mail: g-cziko@uiuc.edu
1310 S. Sixth Street Radio: N9MJZ
210 Education Building
Champaign, Illinois 61820-6990
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