Re: Are there theorems for Agent Based Models

[Dwight W. Read]

I won't cover ground already mentioned by other commentators, but will try
to add a bit to Paul's question about the likelihood of divergence under
his scenario 1 versus scenario 2.  I suspect that it depends on the extent
to which the simulation has non-linearities that lead to large divergences
under small differences in initial conditions. 

If one is talking about simulations of human societies (as is my interest)
it can certainly make a considerable difference as to whether or not one
makes a decision based on the state of all other persons at the end of time
t, or if one acts based on the decisions of other agents at time t+1, prior
to one's one action.  

Paul's question also raises the matter of what information is available to
agents.  How do agents sample their environment?  How often do they update
their sampling?  Shouldn't we also model the agent's access to information?
 This has led me to imagine a simulation in which each agent is totally
asynchronous with respect to all other agents and can independently
"sample" its current environment and then act upon the information it
obtains -- with each agent's action updating the "environment" of all other
agents at the time the action occurs.  

Dwight Read

At 03:27 PM 7/12/99 -0500, you wrote:
>An economist said to me the other day, "where are your theorems? Aren't
>there established results that guide modeling choices?"  I shrugged.
>
>Here is an example of where there is probably a theorem, somewhere,
>somehow.  Consider lots of agents who can observe the behavior of other
>agents within a neighborhood and then adjust their behavior. 
>
>Lately I've noticed you can write this kind of a simulation 2 ways.
>
>1. Assume all agents survey the environment as it was after the last
>time step.  If agents are in time 2, for example, they look around at
>all other agents as they were at the end of time 1.  (Example:Heatbugs
>heat display is updated once per time period, all agents refer to that
>snapshot)  
>
>2. Assume that all agent actions have "instantaneous effects" on each
>other.  If we are in time 2, each agent who observes is able to see all
>agent actions up to that instant.  If in time 2, a long list of agents
>is called upon to act (possibly in random order), each one is aware of
>the actions of the previous agents.  Agents who acted in step 2 before
>are seen and their changes are taken into account.
>
>These 2 approaches yield some initial differences in test models I've
>been working on, but the effects seem to be short term.  
>
>Is there a theorem that says something about the likelihood of
>divergence in observations based on the 2 specifications?
>
>
>
>-- 
>Paul E. Johnson                       email: address@hidden
>Dept. of Political Science            http://lark.cc.ukans.edu/~pauljohn
>University of Kansas                  Office: (785) 864-9086
>Lawrence, Kansas 66045                FAX: (785) 864-5700
>
>
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Prof. Dwight Read
Dept. of Anthropology
UCLA
Email: address@hidden
Phone: (310) 825-3988


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   esp. using Swarm.  For list administration needs (esp. [un]subscribing),
   please send a message to <address@hidden> with "help" in the
   body of the message.
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