All of the responses so far to my posting seem to me to raise
interesting and valid points. Some years ago on this list we had a long
discussion about modelling top-down or bottom-up. The modelling
elements of that discussion were reported as a symposium among Rosaria
Conte, Bruce Edmonds, myself and Malcolm Sawyer in CMOT. I don't want
to revisit the same discussion.
So can we stipulate that SD is a sort of top-down modelling approach
with a static representation of social structure and no direct
representation of interaction among socially embedded individuals?
Nearly 10 years ago, Van Parunak demonstrated that for a particular
social target an SD model yielded different results from those of an
agent based model. As I remember, Van ascribed the difference to the
effect of agent interaction.
I am not aware of any general results in this vein.
If the result were general, then we could define the domain of
application of SD social models as the set (or a subset) of cases where
social interaction among individuals does not affect the macro level
outcome. Similarly, the domain of application of agent based models
would be where social interaction does affect macro level outcomes.
If it is possible to capture agent-based macro level results with an SD
model, then there might well be efficiency gains in using such a model
where scaling up the number of agents is computationally too expensive
with available technology. For this to be a reliable procedure, we
would need some general results about -- or experience with -- the
conditions in which macro level outcomes can be represented as if they
did not emerge from social interaction.
What is not true is the remark that stimulated my original posting. It
is false to say that system dynamics is the modelling method of choice
for social systems. You can only have confidence in its applicability
where there is no effective social interaction among individuals and the
social structure is static.
--
Scott Moss
Professor of Social Simulation
Centre for Policy Modelling
Manchester Metropolitan University
Aytoun Building
Manchester M1 3GH
http://cfpm.org/~scott
(t) +44 (0)161 247 3886
(f) +44 (0)161 247 6802
(m) +44 (0)7740 942564
|