Re: Contra Thompson: problems with the explanation ofexplanations
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Re: Contra Thompson: problems with the explanation ofexplanations
 
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Robert,
===>Robert Holmes wrote:<===
==>Schelling's segregation model is completely misrepresented. The notion that segregation decreases as the individuals' desire to be segregated increases is wrong. Nick - have a play with the Netlogo model! As you increase the "%-similar-wanted" slider, the end-point of the "percent-similar" plot get closer to 100%<===
I DID play with the model! Although you are the last person I would want to go up against on such a geek matter, I think it performs pretty much as the paper describes. In fact, I have it running at this very moment. Percent similar sought is set to 85, average percent similar achieved is running around 50 percent and everybody is unhappy. There is, I think, a dramatic phase change between 70 percent sought and 80 percent sought, in fact, now that I explore it, between 75 and 76%.
Steve, and others: I wonder if this is not a case where increasing the gradient actually DECREASES the structure?
===>Hempels' symmetry of explanation and prediction has been dead and buried for years so really can't be used to support any argument; <===
I think the paper puts it around the other way: that scientific practice supports Hempel, not other way around. "Precisely the same point holds for the other exampleswhich, collectively, serve to confirm, not undermine, Carl Hempel's sixty-year old "Symmetry Thesis" concerning explanatory and predictive power (Hempel 1948)."Can you think of any examples of good scientific theories that do not provide good, clear, expectations of observation?
===>hypothesizing micro-rules in models is actually a perfectly reasonable thing to do;<=== I think I agree; where do Derr and I contradict this assertion?
===>the burden-of-proof should rest with the modeler, not with anyone who dares to disagree with her.<===
Again, not clear what you have in mind, here, or how it stands in contradiction to Derr and Thompson.
Thanks for the comments,
Nick
Nicholas S. Thompson
Emeritus Professor of Psychology and Ethology,
Clark University ([hidden email])
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Re: Contra Thompson: problems with the explanation ofexplanations
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Miscellaneous responses below... -- Robert
On Sun, Feb 1, 2009 at 3:38 PM, Nicholas Thompson <[hidden email]> wrote:
Ah, there's your problem - you can probably only use this model in a limited range of %-similar-wanted. I suspect that at high values of %-similar-wanted there simply aren't enough turtles of the other color to give the target %-similar-wanted. Is that phase change at 75% an artefact of the model? To me, the fact that 75% = (an integer)/(the number of neighbors) -- specifically 6/8 -- suggests that it is an artefact. I'd be REALLY surprised if this phase change corresponded to something in the real world.
No I can't, but that's not actually the symmetry issue with Hempel. I'll stick that in another email once I've had time to think it out properly.
Para 1.5: "Models are chosen or designed to be in accord with what is already known about the phenomenon we are trying to explain."
But consider flocking models: we ignore what is already known (or don't attempt to find out what is known) and instead hypothesize that we can get flock like patterns using the three rules (i) avoid collision (ii) align direction (iii) align speed. In actual fact, from the papers Steve posted earlier this week it actually looks like the rules going through the creatures' brains are (i) pursue thing in front and (ii) escape from the thing behind.
Actually, perhaps your statement is the more accurate: a good model is one that IS based on known micro-rules; making up the micro-rules (which is what we usually do in an ABM) is actually cheating...
Para 1.7: "How can this counter intuitive result come about? Is it just an artifact of all the artificialities in the contrived model? Or does the model point to some fundamental flaw in our thinking about segregation? Modelers would rightfully claim the latter." (emphasis added).
That last sentence is an awfully big claim and is why I'd say that the burden-of-proof rests with the modeler. e.g. I should not be expected to consider your 75% phase change above as a real-world effect - the burden would be very much on you to show that it was not a model artefact.
============================================================ FRIAM Applied Complexity Group listserv Meets Fridays 9a-11:30 at cafe at St. John's College lectures, archives, unsubscribe, maps at http://www.friam.org |
Re: Contra Thompson: problems with the explanation ofexplanations
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In reply to this post by Nick Thompson
Robert,
we seem to be within reach of one another on most points, so I will reply only to this one.
"Ah, there's your problem - you can probably only use this model in a limited range of %-similar-wanted. I suspect that at high values of %-similar-wanted there simply aren't enough turtles of the other color to give the target %-similar-wanted. Is that phase change at 75% an artefact of the model? To me, the fact that 75% = (an integer)/(the number of neighbors) -- specifically 6/8 -- suggests that it is an artefact. I'd be REALLY surprised if this phase change corresponded to something in the real world. "
Well, perhaps you should prepare to be surprised. On your account, one would expect the model to be exquisitely sensitive to the number of turtles. If one increases the number of turtles to 2500, the tipping point doesn't seem to change much. I would further increase the number of turtles, but model limits me to 2500.
Oh, I see! It is the number of neighbors you see as being too coarse-grained. Well, since there are 8 neighbors (in the procedures) one would expect that if this were the problem that .875, not .75 would be the tipping point. But even if .875 WERE the tipping point, why is that an artifact? Assuming that a neighbor is somebody whose bedroom window can be seen from your bedroom window, and given that there is no such thing as half a neighbor, I think "neighbor-number" in the real world is probably pretty coarse grained. One of the things I learned sitting in at the SFI summer school is that discreteness of input is one of the important sources of complexity.
In any case, having implied that I didn't study the model, don't you need, in order to keep the record straight, to amend your original claim that
"As you increase the "%-similar-wanted" slider, the end-point of the "percent-similar" plot get closer to 100%. It does NOT suddenly start dropping. "
Otherwise, you might be accused of Gerry-mandering the data.
Speaking of cups of coffee.
Nick
Nicholas S. Thompson
Emeritus Professor of Psychology and Ethology,
Clark University ([hidden email])
P.S. Oh: Whether or NOT the drop at 75 percent is an artifact or not, the model still points to at least ONE fundamental flaw in our thinking ... the belief that reasonable flexibility in choosing one's neighbors will lead automatically to reasonably integrated neighborhoods. I really do believe we agree much more than you say.
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