Note that Majority Judgment, Range, and even arguably Approval are independent of irrelevant alternatives. Majority Judgment is the clearest; it passes IIA even with simple zero-information strategy. (That is to say, with MJ it is reasonable to vote honestly on an absolute scale, unlike Range or Approval where any reasonable zero-information voter must make sure to normalize their vote.)
I point this out not to disparage Condorcet, but to show that different systems have different, important, advantages. In a world where the best answer to the question "What is the current probability that system X will replace Plurality in the elections that affect me?" is almost universally "Not high enough, and I plan to do something about that," there is no excuse for letting our ideas of which system is best or most probable get in the way of our solidarity with other good systems. In other words, I find it flat-out immoral to say "I agree that your system will be an improvement, but I refuse to say so publicly because that would distract from my system which is better-known publicly." And like it or not, that is what David's argument about expected benefits amounts to. Jameson 2011/11/26 Kristofer Munsterhjelm <km_el...@lavabit.com> > robert bristow-johnson wrote: > > what do you mean: "weight"? rankings are just rankings. if a voter >> ranks Candidate A above Candidate B (independent of what the absolute rank >> values are), all that means is that this voter would vote for A if it were >> a simple two-candidate race with B. and all Condorcet seeks to accomplish >> is to be consistent with that social choice regardless if Candidate C or >> Candidate D were in the race or not. >> >> it's pretty simple: >> >> 1. if a majority of voters agree that Candidate A is a better choice for >> office than Candidate B, then Candidate B is not elected. this imposes >> consistency with the 2-candidate race where we all agree who should be >> elected and why. >> >> 2. the relative merit of Candidate A to Candidate B is not affected by >> the presence of a third candidate, C. in the converse, this means that >> removing any loser from the race and the ballot, that this should not >> change who the winner is. if it does, that loser is a "spoiler". it is >> precisely the motivation for adopting IRV in the first place. >> > > To my knowledge, Condorcet passes IIA whenever there is a Condorcet > winner. If Condorcet winners are frequent, that's a pretty good property. > > That is, if candidate A is a Condorcet winner, and you remove some other > candidate B, A is still the Condorcet winner. If you add some other > candidate C, unless C beats A, A is also still the Condorcet winner. > > Some may not like the tradeoffs Condorcet bring (like failing FBC), but it > bears keeping in mind, I think. While IIA (general spoiler-independence, as > it were) might be too strong to be sensible in the general case, having a > method pass it in certain cases is welcome. > > Advanced methods can go further, as well: a method that passes > independence of Smith-dominated alternatives will not be influenced by > candidates outside the Smith set. > > (Of course, if there's rarely a CW or if the Smith set is usually large, > this doesn't amount to much. Offensive strategy attempts to create cycles > in the strategists' favor.) > > > ---- > Election-Methods mailing list - see http://electorama.com/em for list info >
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