It is my impression that the only situations in which IIAC fails is when there is no majority.
Would it be possible to get around IIAC by adding a two-candidate runoff? Ted On 29 Mar 2012 05:35:47 -0700, Jameson Quinn wrote: > > The Independence of Irrelevant Alternatives criterion (IIA, also sometimes > abbreviated IIAC) is a bit of a silly criterion. Arguably, no system really > passes it. For any ranked system, just take a simple A>B>C>A 3-candidate > Condorcet cycle, and then remove the "irrelevant" candidate who loses to the > winner; any system which reduces to plurality in the 2-candidate case must now > fail IIA. Rated systems can pass, but that means assuming that people will > vote > silly ballots. For example, in approval, ballots with all candidates approved > or all candidates disapproved; or in range, non-normalized ballots. (Majority > Judgment is the only commonly-discussed system where a non-normalized ballot > might not be strategically stupid; but even there, voting all candidates at > the > same grade seems pretty dumb.) > > But of course, because of its role in Arrow's theorem, and because of the > simplicity of definition, it's not a criterion we can entirely ignore. For > instance, it's always going to be a part of the comparison table in wikipedia. > (Which has gotten some updates recently; check it out) > > When it comes to delegated systems like SODA, it becomes even crazier. Is a > candidate "irrelevant" even though their use of the votes delegated to them > was > what swung the election? So, just as Condorcet advocates have defined > "Independence of Smith-Dominated Alternatives" (ISDA), I'd like to define > "Independence of Delegation-Irrelevant Alternatives" (IIDA). A system is IIDA > if, on adding a new candidate, the winner either stays the same, changes to > the > new candidate, or changes to a candidate whom the new candidate prefers over > the previous winner. > > Unfortunately, SODA isn't actually 100% IIDA. The scenario where it fails is a > chicken dilemma where the new candidate pulls enough votes from one of > the??two > near-clone chicken candidates??to shift their delegation order. But it does > meet this criterion for three candidates; that is, a third candidate does not > shift the balance of power between the first two unless they choose to. And I > suspect that you could define a SODA-like system which would meet IIDA, if you > didn't mind adding complications. > > Jameson > > > ---- > Election-Methods mailing list - see http://electorama.com/em for list info -- araucaria dot araucana at gmail dot com ---- Election-Methods mailing list - see http://electorama.com/em for list info