Ventana software support forum

Posted: **Wed Oct 06, 2004 5:58 pm**

Hi All

I am new to System Dynamics and am kind of confused. I am reading
some papers about dynamical systems in social psychology. I thought
it was about System Dynamics but it seems to be something different
and unrelated.

Could anyone of you help me, please?

Why is it so similar in appearance but without relation?

It seems that different people are studying complex systems but they
do not see what others are doing. Or is it just that I am so
uninformed that I do not see the relation?

I need some help! Could someone please tell me the differences or
similarities between Dynamical Systems and Systems Dynamics?

Thank you,

Esteban Ribero
From: Esteban Ribero <eribero@yahoo.com>
Graduate Student
The University of Texas at Austin

Posted: **Thu Oct 07, 2004 2:46 pm**

Hello Esteban,

""System Dynamics"" is a method to study the structure and behavior of
dynamical systems. There are many other methods, but ""System Dynamics""
uses a (by now) fairly standard set of symbols for developing system
diagrams and simulation models. The system diagrams (usually called
""causal-loop diagrams"") depict a hypothesis as to what are the feedback
loops that generate the observed behavior over time. Then, using rate
and level equations (or, in mathematical terms, zero and first order
difference equations), the simulation model is built, and there are a
number of simulation languages to facilitate the process of formulating
the equations and running the simulations.

Generally speaking, if you see graphs of system behavior over time
(based on actual and/or simulated data) *and* you see diagrams with
directed arrows that link system variables as feedback loops -- then you
are looking at a ""System Dynamics"" analysis of a system or process. If
you don't see behavior-over-time plots and feedback loop diagrams --
then you are looking at some other kind of analysis.

To see some examples pertaining to various kinds of systems, visit the
Ossimitz System Dynamics Mega Links Directory:

http://www.uni-klu.ac.at/~gossimit/linklist.php

Do you have any formal training in ""System Dynamics"". There are some
online tutorials, but this is not the kind of analysis that you can
learn easily on your own. I was trained by Willard Fey, who was in the
original group under Jay Forrester at MIT.

Hope this is helpful.
Luis
From: Luis Gutierrez <LTG1979@attglobal.net>

Posted: **Thu Oct 07, 2004 3:06 pm**

In terms of underlying mathematics, Dynamical Systems (DS) and System
Dynamics (SD) are the SAME! They are based upon the concept of ""feedback
control system"", which are usually expressed in differential equations
(derivatives). However in System Dynamics, it is expressed in the form of
integral equations. You know derivatives and integrals are the same thing.
- Dynamical System is expressed in the form of
dX/dt=f(inflow-outflow)
- System Dynamics is represented in
X(t)=X(t-1)+dt(inflow-outflow).
Move the X(T-1) term to left and divide by dt, we have
(X(t)-X(t-1))/dt=inflow-outflow, therefore, the left hand
side is
dX/dt=f(inflow-outflow), which is the same as in DS.
The ""stocks"" and ""flows"" in SD are called ""state variable"" and ""rate"" in
DS.

Why derivatives have been used widely rather than integrals? Well, that's
the way that mathematics have been developed and thus applied especially in
software. However, it is much intuitive and easier to understand any
dynamic behavior in the form of integration.

Hope this helps.

Sangdon Lee
From: sangdon.lee@gm.com

Posted: **Thu Oct 07, 2004 3:32 pm**

Hi Esteban,

My impression is that, mathematically, dynamical systems and
system dynamics are the same thing in the sense that both are
dealing with change through time. However, the usual system
dynamics practice is to model in continuous time, that is, with time
unfolding continuously, whereas, I have found threads of dynamical
systems practice using both continuous and discrete time
formulations.

Although alike mathematically, there are other aspects of SD and
DS that seem to me to be somewhat different.

1) SD typically focuses on changes through time resulting
from the interactions of feedback loops (feedback causality),
whereas, although feedback is usually present in DS models, I
don't think DS emphasizes feedback to the degree that SD does

2) SD practice focuses on identifying operational causal
relationships between variables, whereas I think DS practice is
mostly silent (doesn't care) on whether relationships between
variables should be causal (structural) or correlational (behavioral).

3) Another difference may be the emphasis in SD on the
modeling process (e.g. in SD literature by John Sterman, George
Richardson, Andrew Ford, Khalid Saeed, Barry Richmond, etc...). I
haven't seen a modeling process emphasis in the DS books I have
seen.

4) Another difference may be that SD emphasizes policy
design (policy engineering) as a primary goal of modeling. I don't
think this emphasis is present to the same degree in DS practice.

And there are probably many other differences. I'll be really
interested to see how others respond to your question.

Paul Newton
Instructor
Cornell System Dynamics Network (CSDnet)
Department of Applied Economics and Management
407 Warren Hall, Cornell University, Ithaca, NY 14853
607-255-5230
pcn4@cornell.edu, or
PaulNewton@StewardshipModelng.com
http://www.CSDnet.aem.cornell.edu

Posted: **Thu Oct 07, 2004 6:36 pm**

Hello Esteban,

Your posting interested me, because I am always interested in overlaps
between SD and psychology. I looked on the web, and it appears that this
field you mention represents a relatively new movement among social
psychologists looking at the time course of behavior.

These are suggested texts according to the web:
Vallacher, R. R. & Nowak, A. 1994. Dynamical Systems in Social Psychology.
San Diego : Academic Press.
Guastello, S. 1995. Chaos, Catastrophe, and Human Affairs: Applications of
Nonlinear Dynamics to Work, Organizations, and Social Evolution. Hillsdale:
Lawrence Erlbaum Associates.

It appears that system dynamics and this area of dynamical systems in social
psychology share common mathematical roots. SD has been applied broadly, but
not so much to psychology (which traditionally records behaviour in time
snapshots rather than time series). So that may explain the separate
literature streams.

A couple of years ago, I tried to gather some mathematical psychologists to
look at the overlap, but was limited by costs to only inviting a few speakers
from psychology to speak to the system dynamicists. I think there is
tremendous potential for overlap and discussion if we could only figure out
the logistics.

Another overlap is in cognitive engineering such as described in books like:
Jagacinski, RJ and Flach, JM (2003). Control Theory for Humans: Quantitative
Approaches to Modeling Performance. Mahwah, NJ: Lawrence Erlbaum
Associations.

Elise A. Weaver, Ph.D.
From: ""Weaver, Elise A"" <eweaver@WPI.EDU>
Assistant Professor of Psychology/System Dynamics
Department of Social Science and Policy Studies
Worcester Polytechnic Institute
100 Institute Road
Worcester, MA 01609

Posted: **Sat Oct 09, 2004 1:55 am**

Hello, folks.

Reading Mr. Newton and Ms. Weaver's notes, the first thought that struck me
was ""how many graduate students are paying attention to this thread as they
discern their thesis or dissertation topics?"" Both of the notes suggested
some potentially rich areas within the separate academic worlds and also for
bridging the disciplines.

Good luck to those who follow these paths!

Michael Schwandt
From: ""Michael J Schwandt"" <SCHWANDT@charter.net>
Industrial and Systems Engineering
Virginia Tech

Posted: **Tue Oct 12, 2004 5:16 pm**

Hello Esteban;
Here is one way of characterizing the relations between dynamical
systems and system dynamics: system dynamics is the name adopted by a
community who does work on dynamical systems of certain type (policy
problems of socio-economic or strategic management nature) with a
special focus on underastanding the feedback causes of the dynamics of
concern (or to choose problems that have significant feedback
structures). This particular focus also (naturally) created a set of
specialized tools (causal loop diagrams; stock-flow modeling,
integration emphasis, special validtity tests, interactive gaming, group
modeling, etc).
So, 'dynamical' systems is more general than system dynamics -as WE use
the term. Dynamical systems just means dynamic systems represented by a
set of differential equations. (The origins is somewhat more specific,,
meaning 'dynamic systems in engineering or natural sciences).
Many dynamical system problems are system dynamics problems, but then
again many are not. All sd problems are at least implicitly ds problems
by nature.
I think we all tend to exaggerate the disciplinary boundaries that stem
from terminology and tools. It is more productive to emphasize
terminology about he content/substance of the work (rather than about
methodology/tools).
I try to 'mix' methodology terninology as much as I can. I sometimes use
system dynamics; sometimes dynamical systems; sometimes dynamic systems,
sometimes dynamic systems approach, sometimes systems science/theory,
etc... in the SAME talk, on purpose.
best wishes
Yaman Barlas
From: yaman barlas <ybarlas@boun.edu.tr>

---------------------------------------------------------------------------
Yaman Barlas, Ph.D.
Professor, Industrial Engineering Dept.
Bogazici University,
34342 Bebek, Istanbul, TURKEY
Fax. +90-212-265 1800. Tel. +90-212-359 7073
http://www.ie.boun.edu.tr/~barlas
SESDYN Group: http://www.ie.boun.edu.tr/labs/sesdyn/
-----------------------------------------------------------------------------

Esteban Ribero wrote:

>
>

Posted: **Wed Oct 13, 2004 6:07 pm**

Colleagues,

In terms of practice, I have following observations to make:

1. Model boundary:
In case of dynamical systems modeling, the model boundary is often based on
a specific complex pattern of deterministic behavior, such as bifurcation,
chaos, oscillation, suspected to exist in a system.

In case of system dynamics, we often spend considerable time constructing
custom reference modes addressing complex patterns subsuming growth,
oscillation and policy resilience, which lead us to the model boundary.
Personally, I'd like to see us slicing complex patterns into their generic
components and basing our model boundary on the multiple patterns
encountered in a given problem, but that seems to be a subset of the
general practice.

2. Solution/understanding
In case of dynamical systems modeling, the explanatory process resides in
the solution expressed in terms of initial conditions, parameters,
mathematical functions and time, not in terms of causal relationships
between variables. No variables appear on the right had side of a formal
solution. Complete formal solutions in such forms can be found for simple
models with linear relationships. Solutions to more complex (higher order
and nonlinear) models are often found through repetitive computer
simulations that can identify the parameter ranges over which certain
behavioral characteristics may appear. The solution process in both cases
attempts to cover behavioral implications for all possible parameter sets,
but does not provide a causal explanation of the behavior.

In case of system dynamics modeling, solution manifests in explaining the
dynamic behavior of the model in terms of the changing dominance of its
underlying feedback loops, although behavioral implications for all
possible combinations of parameter sets may not be known. Computer
simulation experiments are the means to getting to the understanding in
terms of the influence of the dominant feedback loops on the behavior and
how this influence can be changed, but there is no prescribed way to
conduct simulation experiments and represent their results, hence modeler
skill is the key to arriving at meaningful results.

I am lately seeing solution related simulations similar to dynamical
decision systems practice being offered in our modeling practice too. I
think this is useful provided an explanation from a feedback perspective is
also given.

Khalid Saeed
From: Khalid Saeed <saeed@WPI.EDU>

Posted: **Wed Oct 13, 2004 7:46 pm**

> who does work on dynamical systems of certain type (policy problems of
> socio-economic or strategic management nature) with a special focus on

Very well said. I know that I propose much better solutions when I focus on
the problem, not on the tool.

You are probably familair with the phrase: ""When the only tool you have is
a hammer, every problem begins to look like a nail.""

Stu Eldred
Acton, MA, US

Posted: **Thu Oct 14, 2004 1:52 pm**

Dear all,

Other contributors have stressed that DS and SD are similar approaches to
similar problem classes relying on different terminology. When I looked at this
some while ago, I came to the conclusion that one of the key distinctions
between these two approaches was the output of the modelling exercise. For
example DS will focus on the existence and stability of solutions, using phase
space investigations and parametrical comparisons, whereas SD tends to focus on
time based output graphs that are more appropriate for the type of problems
under consideration. Many packages support parametric analysis, but I have never
seen phase space analysis in an SD context - but it may be there!

I know of several people who succesfully teach DS type courses using an SD
approach (at least initially) - because of the graphical intuitive nature that
the problem is formulated in and the supporting software that can assist in
analysis.

From: Sean Price <sean.price@thales-tts.com>

Posted: **Mon Oct 18, 2004 7:26 am**

whilst I agree fully with Stu's warning about using our 'hammer' only
when the problem is a 'nail' let's not lose sight of the fact that these
particular nails are ubiquitous. Outside the weird world of quantum
mechanics, I don't think there's any means for causality to operate at a
distance through time or space, except through 'storing' stuff in stocks
- so our world works by accumulating stocks through time, and by
interdependence determining the rate at which those processes occur.

Unless I misunderstand, then, if our problem has anything to do with
things changing through time, it is almost certain to involve
accumulating stocks, and if your hammer doesn't deal with that feature,
it's unlikely to drive the nail. [recognising that SD is not the only
tool that does this]

Kim Warren
From: ""Kim Warren"" <Kim@strategydynamics.com>

Posted: **Mon Oct 18, 2004 12:36 pm**

I hesitate to stir up this sort of hornet's nest but field theories
(electro-magnetic, acoustic,...) do not use material stocks and flows
and they can represent such phenomena as the Doppler effect. However
quantized versions (photons, phonons,...) use flows and densities (i.e.
numbers per unit volume, area).

These theories allow for 'action at a distance' but it is limited by the
speed of transmission -no problem there for SD. The truly weird feature
about quantum mechanics is 'instantaneous action at a distance' -let's
not go there in this discussion, please.

I don't know of any examples in which the classical or quantized field
theories have been translated into SD models.

Joel Rahn
jrahn@sympatico.ca

Posted: **Tue Oct 19, 2004 10:08 pm**

Even though stocks appear to be everywhere in the real world,
as well as important modeling abstractions, I wonder if
their usefulness requires absolute generality. Gravity,
a force fundamental to the way our world works, operates
at a distance in real time through space. Summarizing
the gravitational force as a stock doesn't make sense to
me. However, capturing the outcomes of gravity (e.g.
potential energy) as stocks is probably good enough for
most modeling purposes.

From: ""Ingram,Allan E - PFR"" <aeingram1@bpa.gov>

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