Negative Levels
Posted: Sun Apr 25, 1999 12:55 pm
Id go a step further than Jim Hines and run the risk of distressing my
friend Geoff by suggesting that one should almost never write the
MIN(Level/DT,...) equations he offers.
Best practice, it seems to me, is to try to capture the mechanism in
reality that is responsible for preventing the accumulation from
continuing to decline even though theres nothing left. In practice such
a mechanism would probably be captured, as Jim suggests, in a nonlinear
graphical function (table function) whose slope and shape would reflect
the level of aggregation in the accumulation.
The only type of structure that seems to fit the MIN(Level/DT,...)
formulation is something like pouring water out of a pitcher -- it rushes
out and stops abruptly. Inventories, workforces, production resources,
cash balances, and so on dont have such dynamics, precisely because when
such stocks decline they set up pressures (which should be modeled) that
more gradually constrain their outflows. Some physical systems (like
pitchers) may be captured well by such a MIN formulation, but I would
think that most stock-and-flow processes involving aggregations of people
or resources or averages would be misrepresented if modeled that way. I
think best practice is to avoid formulations like this that look more like
computer programing than social system modeling.
And at the risk of annoying another good friend, Barry Richmond, Ill note
that this issue is the reason I have long objected to the default in
STELLA/kThink that prevents a stock from going negative. (Im greatly
pleased that recent releases allow the user to change the default.) I
think software should not substitute for the thinking one ought to do to
understand the mechanisms, the pressures in reality that are constraining
an outflow when its stock gets small.
But its a debatable topic, worth discussion over a very good beer, as
Goeff and I have done more than once, I believe...
...George
-----------------------------------------------------------------------
George P. Richardson G.P.Richardson@Albany.edu
Chair of Public Administration and Policy http://www.albany.edu/~gpr
Rockefeller College of Public Affairs and Policy Phone: 518-442-5258
University at Albany - SUNY, Albany, NY 12222 Fax: 518-442-5298
-----------------------------------------------------------------------
friend Geoff by suggesting that one should almost never write the
MIN(Level/DT,...) equations he offers.
Best practice, it seems to me, is to try to capture the mechanism in
reality that is responsible for preventing the accumulation from
continuing to decline even though theres nothing left. In practice such
a mechanism would probably be captured, as Jim suggests, in a nonlinear
graphical function (table function) whose slope and shape would reflect
the level of aggregation in the accumulation.
The only type of structure that seems to fit the MIN(Level/DT,...)
formulation is something like pouring water out of a pitcher -- it rushes
out and stops abruptly. Inventories, workforces, production resources,
cash balances, and so on dont have such dynamics, precisely because when
such stocks decline they set up pressures (which should be modeled) that
more gradually constrain their outflows. Some physical systems (like
pitchers) may be captured well by such a MIN formulation, but I would
think that most stock-and-flow processes involving aggregations of people
or resources or averages would be misrepresented if modeled that way. I
think best practice is to avoid formulations like this that look more like
computer programing than social system modeling.
And at the risk of annoying another good friend, Barry Richmond, Ill note
that this issue is the reason I have long objected to the default in
STELLA/kThink that prevents a stock from going negative. (Im greatly
pleased that recent releases allow the user to change the default.) I
think software should not substitute for the thinking one ought to do to
understand the mechanisms, the pressures in reality that are constraining
an outflow when its stock gets small.
But its a debatable topic, worth discussion over a very good beer, as
Goeff and I have done more than once, I believe...
...George
-----------------------------------------------------------------------
George P. Richardson G.P.Richardson@Albany.edu
Chair of Public Administration and Policy http://www.albany.edu/~gpr
Rockefeller College of Public Affairs and Policy Phone: 518-442-5258
University at Albany - SUNY, Albany, NY 12222 Fax: 518-442-5298
-----------------------------------------------------------------------