Ventana software support forum

Jim and others talk about a collection of molecules being assembled. Are
these available to the general public (yet)? I noticed Jims comment that
he was hoping Pal Davidson or his group might put them on the Web, so that
seemed to imply that they would become available in the future, but "hope
springs eternal ...."

Regards,

Bill


--
Bill Harris Hewlett-Packard Co.
R&D Productivity Department Lake Stevens Division
domain: billh@lsid.hp.com M/S 330
phone: (206) 335-2200 8600 Soper Hill Road
fax: (206) 335-2828 Everett, WA 98205-1298

[Eberlein: The molecules in their current state will be made available on
the web in the near future (read 2 weeks). I will post a message
with details ]

Jim Hines clarification of the difference between archetypes, generic
structures and molecules was helpful. In our paper in System Dynamics
Review 12(2) - "Reinterpreting Generic Structure: Evolution, application
and limitations of a concept" - Chris Smart and I propsed that there were
three different versions of the generic structure concept:

1) Cannonical Situation Models: fully specified simulation models that
generate a range of behaviour modes. Such models attemp to represent a
general class of situations and explain the different behaviours that are
observed in different examples. The urban dynamics model is an example.

2) Abstracted Micro-structures: smaller simulation models which produce one
mode and which are used to construct larger models. We gave the overshoot
and collapse strucure as an example.

3) Counter-intuitive System Archetypes: the Meadows idea, via Senge, of
explaining an odd system behaviour or an unexpected intervention point
using a very simple causal loop diagram which brings to life a story or
stories.

In the paper we talk about the different uses of the three types and
about their stengths and weaknesses.

END OF PLUG

I like the molecule idea. They sound as though they exist at a level
slightly below our Abstracted Micro-structures but may overlap. They feel
like the list of rate formulation in Chapter 4.1 of Richardson & Pugh. I
would like to hear more about this idea and will be at the talk in Boston.
However, in Boston, as part of my presentation, I will also be launching
an attempt to record and document the known generic structures (all 3
types!). I think that we need to be clearer about concepts such as these or
we will end up chasing illusions or fabulous beasts that may not exist. We
also need to make a better job of formaly recording the discoveries that
have been made. It looks as though Jim is on to this second point as well,
so we will get together and see how molecules fit in and try to trigger
some synergy.

================================================================
Dr. D. C. Lane
D.C.Lane@lse.ac.uk
Operational Research Dept.
London School of Economics and Political Science
Houghton Street
London WC2A 2AE
United Kingdom


Tel: (UK) 0171-955-7336
Fax: (UK) 0171-955-6885

As David Lane suggested, molecules seem to refer to a class of generic structures slightly below his
Abstracted Micro Structures. Abstracted Microstructures are intended to generate a mode of behavior.
Behavior is not really the central point for molecules, structural representation is. At one extreme,
molecules might not even have any dynamic behavior because they lack both loops and integrations. The
"soft if-then" (contributed by George Richardson) is an example. This molecule lets you switch
gradually from one alternative to another as conditions gradually change (e.g. swtiching from
enthusiastic support to bitter opposition to some proposal as information on its effects becomes
clearer). At the other extreme are larger-but-still-small structures such as the "inventory,backlog,
shipping" molecule. This molecule actually does have dynamics, but its role as a molecule is to solve
the problem of coordinating the draining of two different stocks -- e.g., a stock of orders and a stock
of inventory.

An important aspect of the effort to create a compendium of molecules is to also show their structural
similarities one to another. For example, a coflow can be seen as a smooth that has added structure
defining the time "constant". ("Constant is in quotation marks, because this concept, which is
constant in the smooth, is a variable in the coflow.)

David Lane had the idea of creating a Web site to serve as both a collection and a distribution point
for the different kinds of generic structures. Pal Davidsons group in Bergen Norway has the competence
to create such a site, and we have hopes that they will.

Regards,
Jim Hines
JimHines@Interserv.Com
LeapTec and MIT