REPLY Structural or Behavioral Theory (SD6254)

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Posted by  "John Gunkler" <jgunkler at sprintmail.com>

Kim Warren's post (no, of course you didn't miss the point -- you nailed it
directly to the wall as usual!) reminds me of a concern I had when I first
saw an SD model.  It stemmed from the following question:
 
Is there only one feedback structure that can give a good fit to real-life
data (in a suitably complex situation)?
    -- Because, if there can be more than one, how would we ever know which
one to trust and use?
 
But I believe in Kim Warren's example (falling sales and its dynamics) he
has shown the answer.  Whatever it is that is structurally different between
two models producing the same behavior mode, there should be real-world
evidence (data) about.  So, as he explained in his example, if one model
uses falling customer win rates and the other uses increasing customer loss
rates to produce the behavior of interest, we can go look at (or collect)
data to see which is actually occurring [or if both, or neither, is
happening].
 
No matter how much we then disaggregate our model, the same kind of thing
will happen.  First, we must create a structure that produces the behavior
mode.  Second, if we have more than one structure that does this, we look at
real-world data to see which structure "does what the real world does for
the same reasons."  And, as Kim says, that structure is "right" at its level
of detail/disaggregation.
 
I cannot think of a possible exception to this.  Anything we put into the
structure of our models must have a real-world counterpart; that
counterpart's actions must be measurable (even if some things may be
difficult to measure); so Kim's method will work every time.
 
And to those, probably not in the SD field, who question the measurability
of particular stocks and flows, I simply refer them to E.L. Thorndike's
classic remark:  "Whatever exists, exists in some quantity."

John
Posted by  "John Gunkler" <jgunkler at sprintmail.com>
posting date  Sun, 11 Feb 2007 11:53:56 -0500