REPLY Meaning of Stock/Level (SD6947)

[SDMAIL Jack Harich]

Posted by  Jack Harich <register at thwink.org>

> any variable that can change only over time, needs to be treated as a 
> stock.

Perhaps this principle is incomplete. As stated it would mean, for 
example, that the temperature of an object must be modeled as a stock. 
Temperature cannot change instantaneously. It can only change gradually, 
over time. The same could be said for intelligence, trust, political 
power, etc.

Maybe it's more like nouns versus verbs. Variables that are verbs 
(rates) have a unit per time period, like persons/year. Nouns have only 
the unit, like persons. Verbs cannot be a stock, but nouns can 
optionally be a stock. It's just a question of which nouns you want to 
emphasize in the model's structure by making then stocks.

That such contentious principles have not been nailed down would seem to 
be an indicator of how young the field of system dynamics remains.

Jack
Posted by  Jack Harich <register at thwink.org>
posting date  Sun, 20 Apr 2008 08:19:04 -0400