Re: Simulating Individual B

[cgl]

Mike Brown writes:

> I think Paul's question raises two important points - though they may be more
> about the organization of science than about the inherent difficulties of
> SWARM or any other ABM (Thanks, Chris. I like that acronym better too.)
> Ecologists are concerned with aggregations of individuals; they have not had
> to know as much about individual species as, for example, zoologists.
> Similarly,  microeconomists focus on the behavior of individual firms and
> macroeconomists on the aggregate behavior of the economy, etc.

Excellent point, and I do think that it is more about the organization
of science than inherent problems with models...

In many scientific fields (but ecology and economics will serve nicely
as examples), there are those (usually theorists) who study the
macro behavior, and those (usually field workers) who study the
micro behavior - and the two communities rarely interact with 
each other. The models built by each community often simply
make enormously simplified assumptions about the other's results
as the starting point and/or boundary conditions for their own
models. One of the main reasons I am interested in these ABM
models is that they provide an opportunity to close the loop
and look at the way in which the behavior treated by the 
theorists at the macro level is grounded in the micro-level
behaviors treated by the field workers - it all has to come 
together here - the macro behavior should emerge out of the 
micro behavior, as it does in the natural system. Thus
these ABM models tend to be "meso"-scale, bigger than the
small slices of systems typically treated by the field-workers,
but necessarily smaller than the full scale macro-system: just
big enough to study the way in which the macro emerges out of
the micro, but not so big that it rivals the real system in 
all its unwieldy scale...

  It is precisely because these models can serve as a common
venue for both the macro and micro communities in a discipline
that they are of so much interest (and, I think, importance).

>  First, the ecologist and macroeconomist have to
> "ratchet down" and study the behavior of individual entities. Moreover, they
> have to know enough about individual behavior to determine which specific
> behaviors might be relevant to the question under investigation. While this is
> not an insurmountable problem, it does demand a new focus for researchers.

> Second, there are some disciplines where data on the behavior of individual
> "agents" simply has not been studied. To take a bad example, look at
> economics. Macro studies the behavior of the aggregate, and micro the study of
> the firm -- but who has been looking at the behavior of the consumer? We have
> been able to make a "rational actor" assumption for so long that we have not
> bothered to collect data about the real-life behavior of induividual
> consumers.

> For these reasons, I think Paul is very right -- modeling and validating the
> behavior of individual entities can be very tricky. 


  Right again - however I consider this as a feature rather than a bug!

  I think it gets at the essence of what "modelling" is all about...
  The term "model" means a lot of different things to different people.
  However, I find that the most useful characterization of a model is
  as an artifact that you construct that helps you think about the
  system under study - this can range from obviously over-simplified
  models that simply help you understand what data you still lack, and
  what parameters might be the interesting ones, to a full blown
  *theory* that claims to explain exactly how the real system works.

  The point is that in building these ABM models, often the first 
  thing we learn from them is that we have to go back to the real
  system with new questions....but this is good! It is very useful
  to have a tool that provides you with a new way to look at your
  problem and which generates new questions about it.

  Mike (and Paul) may be right about the general lack of 
  data, although I'm willing to bet that some of this perception is
  due to the lack of communication between the macro and micro
  scale research communities. But, I also think that this is 
  because we haven't been thinking about some of these natural 
  systems in the right way, I think ABM models provide the proper 
  venue to help us think about the models in the right way, and if 
  the result of that help in thinking is to force us to collect new 
  data about the system, then that's a useful and necessary lesson....

  So - again, I don't think of this as a problem, but a feature
  of these models......and I don't think of it as unique to 
  ABM models, per se - I think new tools often offer us new
  "opportunities" to collect data...

  Chris Langton



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