Arenas VBA interface would allow its powerful discrete functionality to be
linked with any SD package that also provides a VBA interface (I dont know
if any do).
As others have mentioned, STELLA provides some discrete capability that
allows the construction of combined models. In my experience, while the
use of some discrete logic often improves user acceptance due to better
apparent realism (certainly an important benefit), the discrete logic
rarely changes the fundamental behavior pattern of an inherently
feedback-driven system. My students are always drawn to the discrete
constructs despite my insistence that they avoid them initially. Some
years ago, I challenged my students to replace all their discrete logic
with [roughly] equivalent (and simpler) continuous logic. To their
amazement, in not one case did it have an appreciable impact on the
fundamental behavior PATTERN of their model.
Am I saying that discrete logic is not useful? Not at all!! What I am
saying is that in those cases where the behavior of the system under study
is predominantly FEEDBACK-driven, the incorporation of discrete logic tends
to serve more as a fine-tuning mechanism.
On the other hand, for systems where the fundamental driving concern is the
UNCERTAINTY in the various processes in the system, I believe that a
discrete formulation is essential. For relatively simple processes, I have
found STELLA to work reasonably well. However, it does not allow one to
model and track individual entities with multiple attributes; and its
statistical data collection and analysis capabilities are limited. Hence
the need for discrete system simulation packages such as Arena, SLAM,
ProModel, Simscript, GPSS, and others. I do not agree with Bob Walkers
assertion that SD packages are inherently "far more powerful" than DSS
packages. I believe that both are very powerful and useful...but for
different problem situations.
In situations where feedback AND uncertainty both play a major role in
determining the behavior of the system, one would want a simulation package
that does both equally well. Of the packages I am familiar with, the one
that comes the closest to doing this is Extend from Imagine That, Inc.
But, at a price. In my opinion, Extend is not nearly as good for
visualizing and modeling pure feedback systems as the other SD packages,
nor is it as good for pure discrete system simulation as the best of the
DSS packages. The other problem is that although the basic module
libraries provided with Extend do allow one to construct complex, combined
discrete-continuous models, the execution speed is very slow (one of our
graduate students is addressing this second issue as part of his
dissertation research).
I have room in my toolkit for a variety of tools: a good SD package
(STELLA, Vensim, or Powersim), a good general-purpose DSS package (Arena is
my choice), a good process-oriented package (Extend or ProcessModel), and a
manufacturing-oriented DSS package (ProModel or Arena).
Wayne Wakeland
Adjunct Professor, Portland State University
Portland, Oregon
wwakeland@uswest.net