generic structure vs. archetypes vs. molecules

[jimhines@interserv.com]

"Archetype", "molecule", and "generic structure", are terms that are
cropping up on this list and should not be confused.

1. An "archetype" is one of the relatively small number of dynamic learnings
that have emerged in our field over the past 30 years. Archetypes are
complete little feedback structures that contain a lesson. They are commonly
represented as causal loops. Although Dana Meadows put together a list
earlier, the list that most people refer to these days is from appendix 2 of
Peter Senges __The Fifth Discipline__. This list provides a terminology
that has gained acceptance. The archetypes that Peter lists are balancing
process with delay, limits to growth, shifting the burden, shifting the
burden to the intervenor, eroding goals, escalation, success to the
successful, tragedy of the commons, fixes that fail, growth and
underinvestment. There is no accepted set of simulating models that
correspond to these loops, although a few people have put together such
models. I put together one such set of models, an effort funded by Gemini
Consulting under a contractual arrangement with Ventana.

2. Molecules are common building blocks of system dynamics models. They
represent modeling wisdom, NOT feedback lessons. Molecules are commonly
represented as stock and flow structures. The first were built into Dynamo
as functions (e.g. smooth, delay). High Performance Systems provides (or
provided) a number of moelecules in their documentation to iThink/STELLA. A
more comprehensive set, drawing on the High Performances start, has been put
together (and is being put together) by a number of people including me,
George Richardson, Bob Eberlein, James Mellish, Dewey Johnson, and Liz
Krahmer. The effort was partially funded by Eastman Chemical Company. This
effort is continuing and promises to provide the field with a cohesive set of
bilding blocks with a single terminology. I invite anyone with a "molecule"
to send it to me, we will add it to the growing list (with proper
attribution, of course). Examples of molecules include smooths, material
delays, draining processes, coflows, protected levels, aging chains, and
many others.

3. "Generic Structure" has a more general meaning. People have used the
term to refer to complete models (e.g. Forresters Market Growth as Influenced
by Capital Investment), Archetypes, and Molecules. Basically, "generic
structure" seems to be any feedback structure or stock-and-flow structure
that crops up repeatedly in our field.

Hope this helps.

Regards,
Jim Hines
LeapTec and M.I.T.
JimHines@Interserv.Com