REPLY Meaning of Stock/Level (SD6898)

[SDMAIL Bob Eberlein]

Posted by  Bob Eberlein <bob at vensim.com>

Hi Everyone,

This discussion is interesting, but seems to have a lot of force in a 
direction I consider extremely misleading, if not simply wrong.

First, the whole concept of a stock, level or state variable is an 
abstraction, so there is no absolute test of appropriateness. Just as 
Newtonian physics works very well on slow moving macroscopic objects so 
are stocks and flows a good way to come at a variety of problems. Any 
concept that can be quantified in some sensible way is a candidate for a 
model variable, whether or not it is practically measurable. And any 
variable that can change only over time, needs to be treated as a stock. 
In fact, even in everyday language, we speak of things such as building 
anger, rising emotions, falling into despair. Stocks every one.

The direction that is troubling is the belief that models containing 
these types of variables should be treated as qualitative. Much of the 
power of System Dynamics comes directly from the ability to combine 
concrete measured concepts with intangibles. For example, many very good 
project models include worker morale right along side the nuts and bolts 
of getting work done.

That duality is a strength. Including it is vital to capturing the true 
dynamics of problems being addressed. Combining tangible and intangibles 
also provides a mechanism to measure the values of those intangibles 
indirectly through the model. The contribution of that indirect measurement 
can be tremendous.

It is not appropriate to think that every element of a model needs to be 
validated against existing observations. Many important things are not 
observed, or at least not observed well. It is useful to validate every 
element of a model against some understanding of reality based on what 
is observed and everything else that seems important. The inclusion of 
soft variables does not take away from our ability to compare the hard 
variables to measurements. The appropriate inclusion of soft variables 
in a nuts and bolts model makes the model that much more valuable.

Sorry for the venting - but I had to get that out before I exploded.

Bob Eberlein
Posted by  Bob Eberlein <bob at vensim.com>
posting date  Thu, 10 Apr 2008 09:20:36 -0400