REPLY Structural or Behavioral Theory (SD6244)

[System Dynamics Mailing List]

Posted by  George Richardson <gpr at albany.edu>

> SD has been regarded as a useful framework for theory development.  Why is a
> theory created under this paradigm often called a theory of  behavior? Why
> is the same theory not called a theory of structure?

I think we do tend to think of model structure as a "theory."  But it  might 
be more precisely termed a "hypothesis."  That's the sense we  mean when we 
posit an initial "dynamic hypothesis" -- a simple stock- and-flow/feedback 
structure put forward initially as the key  structure underlying some pattern 
of dynamic behavior.  (I think the  original source for the idea of a "dynamic 
hypothesis" is Randers, J.  (1980), Guidelines for Model Conceptualization. 
Elements of the  System Dynamics Method. J. Randers, Cambridge MA, Productivity 
Press:  117-138, also reprinted in Modelling for Management.)

Simulation helps us test whether a structural theory (hypothesis)  fits 
observed behavior.  If the structure passes all sorts of  plausibility tests 
that try to match the structure to what people  think the key structure of the 
real system is, and if simulations  show that the structure is capable of 
generating the range of  behaviors people have observe or expect in real 
data, then we gain  confidence in the structural theory as a plausible theory 
of  behavior.  If we like the test results we might find ourselves saying  
"our theory of structure looks pretty good," that is, we can't seem  to find 
plausible tests that lead us to want to reject the structural  hypothesis.

The duality here -- theory of structure / theory of behavior -- thus  pops up 
in our tests for building confidence in a model.  We have  tests of model 
structure and tests of model behavior.  (See the  lovely diagram in Saeed, 
K. (1992), "Slicing a complex problem for  systems dynamics modeling," System 
Dynamics Review 8(3): 251-262].)

Still, I think we are being our clearest when we say we "hypothesize"  
structure and test that hypothesis as a "theory of behavior."

George P. Richardson
Chair of public administration and policy
Rockefeller College of Public Affairs and Policy
University at Albany - SUNY, Albany, NY 12222 
Posted by  George Richardson <gpr at albany.edu>
posting date  Tue, 6 Feb 2007 09:40:53 -0500