From: ks3y+@andrew.cmu.edu (Kenneth L. Simons)
Newsgroups: sci.systems
Subject: Re: Beyond the Limits and world modelling
Date: 21 Aug 92 17:56:50 GMT
Article-I.D.: andrew.4eZNvWW00iUz05l28q
Organization: Doctoral student, Social & Decision Sciences, Carnegie Mellon, Pittsburgh, PA

Don Taylor (dont@klic.rain.com) writes,

>I saw a copy of Beyond the Limits (which describes updates to the models used
>in The Limits to Growth) in the local bookstore.  It had a note in the back
>that the model of the world described is available to run under Dynamo or
>Stella.
> 
>Is there an inexpensive version of either of these simulation engines
>available?  Perhaps a student or educational version?
> 
>If not, is there another simulation engine the model could be ported to
without
>a great deal of effort?

Copies of model for "Beyond the Limits" are available for STELLA from
the Laboratory for Interactive Learning (address below).  There is some
charge; I don't know how much.  They must have the Dynamo version too,
but you might have to implement by yourself the changes that they made
in doing the "Beyond the Limits" study.

The modeling program STELLA II is available from High Performance
Systems, Inc. (address below), or from mail order Macintosh software
firms.  STELLA II is virtually identical with another product, iThink,
sold by the same company.  The manual for iThink is geared toward people
who use spreadsheets, while the manual for STELLA II hops directly into
systems thinking.  Call the company for details.  You can use the Beyond
the Limits model in either STELLA II or iThink.  The company used to
have, and may still have, academic discounts.

For ease of use and for educational purposes, I definitely prefer STELLA
II over Dynamo.  It has a visually-oriented interface.  You can lay out
the model on the screen and see it as a diagram.  Unfortunately, STELLA
ONLY RUNS ON MACINTOSH COMPUTERS.  If you do not have access to
Macintosh computers, then you can use Dynamo (from Pugh Roberts, Inc.;
address below), or other modeling programs available for PCs and
workstations may be able to read and use Dynamo models.  Other programs
include DYSMAP2 and Vensim (both of which are described in a recent
issue of the "European Journal of Operational Research," vol. 59 no. 1,
pages 203-209 & 216-219) and SimTek (available from ModellData A/S and
in 1990 priced at $280 / $168 educational, address below).

A Macintosh program called "Beyond the Limits" will be available this
fall for a low price from the Laboratory for Interactive Learning
(address below).  It requires that you have HyperCard version 2.0 or
newer (free with all new Macintosh computers).  This program is much
easier to use than STELLA, Dynamo, etc.  It lets you simulate the
"Beyond the Limits" model, draw graphs over time of any variables in the
model, and change any of the factors that the authors of the book
change.  It includes some introductory discussion about how to use the
model and about real-world goings-on that are background knowledge for
the ideas in the model.  It also discusses what's going on inside the
model when you make changes to it.  To change the model in ways not done
in the book, you have to either buy the STELLA (or Dynamo) version of
the model, or wait a while for the next version of this "Beyond the
Limits" program.  (Future versions will probably allow people to change
the model to address criticisms, additional influences not in the
original model, and misconceptions.  Hopefully, future versions will
allow people to view diagrams of the model, then click on any part of
those diagrams for an explanation and equation of that part, and if
desired change any constant or "table function."  Diagrams of the model
that don't let you click for information are included in the
about-to-be-released version.)

If you want to pursue this topic you will of course need to read the
book "Beyond the Limits" (Chelsea Green Publishing Co., address below). 
It does no good to play around with the model unless you first
understand the point of the model and how the authors use it and
interpret it.  If you want to delve seriously into the model, you may
also want the book "Dynamics of Growth in a Finite World" (Productivity
Press, address below).  This book documents the model, for each part
giving a summary of relevant scientific work, and explaining why the
authors formulated the model the way they did.  If you need references
to get acquainted with modeling of dynamic systems, let me know.

Good luck and have fun!

Ken Simons.

ADDRESSES

Laboratory for Interactive Learning
Hood House
University of New Hampshire
Durham, NH 03824
Phone: (603) 862-2186
Fax: (603) 862-1488

High Performance Systems, Inc.
45 Lyme Road, Suite 300
Hanover, NH 03755
Phone: (603) 643-9636
Fax: (603) 643-9502

Pugh Roberts, Inc.
41 William Linskey Way
Cambridge, MA 02142
Phone: (617) 864-8880
Fax: (617) 661-7418

ModellData A/S
N-5120 Manger
Norway
Phones as of 1990: +47 5 19 57 67
.................  +47 5 37 40 09
Fax as of 1990: +47 5 37 35 00

Chelsea Green Publishing Co.
(somewhere in Post Mills, Vermont)
1-800-639-4099

Productivity Press
(somewhere in Cambridge, Massachusetts)
1-800-274-9911