REPLY Meaning of Stock/Level (SD6902)

[SDMAIL John Gunkler]

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

I think Tom Fiddaman has helped clarify one of Alan's points -- namely, that
what we often call "stock and flow" representations are also called "state
and rate" representations, and that "states" need not be material.

But is there an underlying, and quite legitimate, issue here of
"measurement" (which I think Fiddaman is referring to when he mentions
"representation of a soft variable")?

In SD, and in most of the other fields I've practiced in, there is a serious
problem with allowing people to include concepts in their thinking that have
not been "operationalized."  In the Six Sigma process improvement
methodology, for example, there is a requirement that everything we include
in the analysis have "operational definitions."  By this is meant, simply,
that we must specify a procedure ("operation") for measuring the concept --
and it is this "measurement" result that will always enter into our
equations.

When SD modelers include "soft variables" without specifying how they are to
be measured (or operationalized), I agree with Alan that the resulting model
is problematical.  This does not necessarily mean, as Jay Forrest and others
point out, that the models are useless.  They may help people clarify their
thinking to a degree.

But I am reminded of a cartoon which showed three people on a basketball
court -- one of whom was a short man with seven legs.  The second person was
saying to the third, "Coach, I found that seven-footer you wanted."  

Misinterpretations caused by failing to operationalize the definitions of
soft variables could lead to results equally absurd, don't you think?

John Gunkler
Posted by  "John Gunkler" <jgunkler at sprintmail.com>
posting date  Thu, 10 Apr 2008 11:53:06 -0400