Ventana software support forum

Posted by Kazem Yaghootkar <yaghootkar@yahoo.com>

Hi Everybody

The website of the society introduces SD as:

""System dynamics is a methodology for studying and managing complex
feedback systems, such as one finds in business and other social
systems""

This definition is emphasising that SD is the methodology that is
for systems with feedback. However in many instances I have
developed models which uses stock and flows and are very helpful
for understanding the system behaviour but they do not have
feedback. Are these models still System Dynamics models?

Kazem Yaghootkar
Manchester Business School
Posted by Kazem Yaghootkar <yaghootkar@yahoo.com> posting date Mon, 18 Jun 2007 16:58:08 +0100 (BST) _______________________________________________

Posted by Fabian Fabian <f_fabian@yahoo.com>

Kazem,

If the operational reality you are trying to model is without feedback
(assuming you're modeling a slice of reality which has that kind of structure),
then you'll find yourself having a valid system dynamics model, even though it
is without feedback.

With the simplest generic example, if you model a constant inflow which
contributes to the growth of a stock, you will obtain a - dynamic - linear
behavior of the stock variable. And yes, it will be a valid system dynamics
model.

Of course it would be valid contrasting it against a very thin slice of
reality.

Be well...

Fabian Szulanski
Director- System Dynamics Centre
Buenos Aires Institute of Technology
Posted by Fabian Fabian <f_fabian@yahoo.com> posting date Tue, 19 Jun 2007 06:10:37 -0700 (PDT) _______________________________________________

Posted by Martin Schaffernicht <martin@utalca.cl>

Hi Kazem,

maybe you'd find some discussion of this in ""Why has feedback systems thinking struggled to influence strategy and policy ..."" by Kim Warren, in Systems Research and Behavioral Science; Jul/Aug 2004; 21, 4; pg. 331

In this article, Kim Warren argues that even without feedback, the interplay between in- and out-flows into and from a stock are dynamically complex and that feedback is not always neccesary/useful.

It is certainly true that the relationship between flows and stocks is ""surprising"" to most people; that's why there is quita an amount of reseach&publications about ""stock-and-flow"" thinking (you may refer to the list indicated by John Sterman in a recent reply on this list).

If there is no link from the stock to the flow, this menas you assume that the stock's values have no influence on what determines the flow.
If there are good reasons to believe this, why not? Also, such elementary models are useful in teaching.

Still, for anything beyond Fabian's ""very thin slice of reality"", Jay Forrester has always argued that these influences do exist. So the doubt remains...

best,

Martin Schaffernicht
University of Talca (Chile)
Posted by Martin Schaffernicht <martin@utalca.cl> posting date Fri, 22 Jun 2007 12:32:34 -0400 _______________________________________________