REPLY Meaning of Stock/Level (SD6897)

[SDMAIL jayforrest]

Posted by  <jayforrest at jayforrest.com>

Alan highlights an issue that is, in part, why I moved more to qualitative
system dynamics (or perhaps more precisely, qualitative method involving
logic from system dynamics) for I, as a futurist, routinely deal with
situations where intangibles are significant and fuzziness clouds
quantification. For me, quantification of intangibles raises substantial red
flags and usually demotes the output to speculative status. 

While I work primarily with causal loop or influence diagrams, I would
suggest there is value in modeling intangibles in conventional SD format for
the explicit implications of the SD language provides increased clarity and
basis for inference and analysis over purely causal diagrams. In Alan's
example, he lists Trust. In a pure causal loop diagram there may be some
ambiguity as to whether it behaves as a stock or not. When represented as a
stock it implies a sense of memory and stabilizing influence - but a
precarious one that can disappear rapidly in the event of a catastrophe
(like a lake when a dam breaks). It also implies (which should be in the
model) a flow of trust in and trust out with some coflow with events. While
I find that causal loop diagrams facilitate discussion of such issues, the
rigourous logic of SD contributes to stronger logic when contemplating such
issues. On the other hand, I don't feel I need to quantify the model to
learn from it. Rather it is for internal checks for logic and consistency
and for communication with others who understand at least to some degree the
SD logic.

In summary,  I agree with Alan that quantifying intangibles invites serious
problems both in the nature of the quantification, the boundaries of the
model, and the interpretation of the output. In my opinion such models
deserve careful consideration and potentially qualification before accepting
the output or implications. But I also think that representing intangibles
in stock/flow form offers insights in both contemplation and communication
that make qualitative modeling in this stock/flow form useful. 

Regards!
Jay Forrest 
Posted by  <jayforrest at jayforrest.com>
posting date  Wed, 9 Apr 2008 15:11:54 -0500