REPLY Meaning of Stock/Level (SD6886)

[SDMAIL Alan McLucas]

Posted by  "Alan McLucas" <A.McLucas at adfa.edu.au>

Ricardo's comment reinforces the dilemma.
The concern I still have is that whilst we might legitimately use
stock-and-flow diagrams to enable conceptualization, the strict
mathematical relationships that control those stocks and flows simply do
NOT apply to soft variables such as "brand loyalty".  This means that
the conceptualisation cannot be translated through to building a
quantified stock-and-flow representation of "brand loyalty", or other
soft variables or intangibles that do not adhere to the mathematical
rules for integration.  This is the issue raised by Geoff Coyle in 1999
in Wellington, NZ.  Clearly we still struggle to establish the limits to
qualitative SD.  

I thank Tom for pointing out that we need to create proper
representations soft variables. 

Certainly, we need to include soft variables and intangibles in our
models.  Jay Forrester tells us that not to include them can only
guarantee one thing, our model will be wrong.  He also tells us that any
model we build must produce dynamic behaviours that appear like those we
encounter in the real-world problem.  Further, and most importantly, the
algebraic relationships in our models must be derived from real-world
cause and effect.  

This last point creates explicit demands for validating models.  This is
my main concern: we must be able to validate the behaviour of the model
and demonstrate how it relates to the real world, and this must be so
for all aspects of our model, including our representations of soft
variables.  

I am yet to be convinced that representations of soft variables that
often appear in SD models are routinely and comprehensively validated,
rather than just appearing to be plausible.  I would dearly love to be
proven wrong.  If I am wrong, then this creates an opportunity to
compile a compendium of models that have been validated and, hence, we
can take as being proper representations of soft variables.

Dr Alan McLucas
Posted by  "Alan McLucas" <A.McLucas at adfa.edu.au>
posting date  Thu, 10 Apr 2008 08:14:23 +1000