There is a world (view) of difference between modeling in spreadsheets and
system dynammics modeling. The essence of the difference is that
spreadsheets capture DISCRETE year-to-year (or time unit-to-time unit)
changes while system dynamics simulations attempt to capture a CONTINUOUS
view of change.
One can build an exact replica of a system dynamics model in a
spreadsheet, but then one would have to have (in the most likely format) a
column for each Time Step (DT) in the simulation. For a time step of,
say, .25 year, one would need four columns to move from one year to the
next. Usually, people dont do that, preferring to let the successive
columns be the successive years (or whatever the time unit). Spreadsheets
in which successive columns represent successive time units are capturing
difference equation models, not continuous time models.
It can make a big difference. In a spreadsheet model representing a
difference equation structure, one does not have to obey our presumption
of at least one stock in every feedback loop. A variable can be set equal
to some variable "in the last period," and the loops that result may not
have any stocks in them. In a system dynamics model, there is no "last
period" -- the perspective is of a continuous system, with time flowing
continuously. Forrester had quite a bit to say about this in Industrial
Dynamics.
In the haands of careful experts, I think either modeling perspective can
yield robust results. But the discrete approach that most spreadsheet
users would produce is frought with peril. In Feedback Thought in Social
Science and Systems Theory, there is a lot on discrete and continuous
views. If you can find the book, look up "continuity and discreteness,"
"continuous point of view," "discreteness," and "events." Its a sobering
and instructive set of stories...
...George
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George P. Richardson
G.P.Richardson@Albany.edu
Rockefeller College of Public Affairs and Policy Phone: 518-442-5257
University at Albany - SUNY, Albany, NY 12222 Fax: 518-442-5298
http://cnsvax.albany.edu/~gr383/
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