On 03/14/2013 06:45 PM, robert bristow-johnson wrote:
IRV will prevent a true spoiler (that is a candidate with no viable chance of winning, but whose presence in the race changes who the winner is) from spoiling the election, but if the "spoiler" and the two leaders are all roughly equal going into the election, IRV can fail and *has* failed (and Burlington 2009 is that example).
If you think about it, even Plurality is immune to spoilers... if the spoilers are small enough. More specifically, if the "spoilers" have less support in total than the difference in support between party number one and two, Plurality is immune to them.
So instead of saying method X resists spoilers and Y doesn't, it seems better to say that X resists larger spoilers than Y. And that raises the question of how much spoiler-resistance you need. Plurality's result is independent of very small spoilers. IRV's is of somewhat larger spoilers, and Condorcet larger still (through mutual majority or independence of Smith-dominated alternatives, depending on the method).
Some rated methods are claimed to pass IIA outright and so not be affected by "spoilers" no matter how large they are. That is true - as long as the voters submitting the ratings are comparing the candidates to a common standard rather than to each other.
So IRV advocates can say that IRV will prevent a spoiler from spoiling the election, according to some definition of spoiler. So does Plurality for a less useful definition of spoiler. In any case, it seems that IRV's resistance against candidates that don't win isn't good enough.
It's like reinforcing a bridge that would collapse when a cat walks across it, so that it no longer does so, but it still collapses when a person walks across it. Cat resistance is not enough :-)
It would be really useful to know what level of resistance is enough, but that data is going to be hard to gather. It also depends on your bar. If you're perfectly content with Australian two-and-a-half party rule, then IRV is good enough. If you want resistance even in a game theoretical situation where everybody can communicate with everybody else and votes are purely instrumental, then no ranked voting system is going to work... and so on.
And beyond that we have even harder questions of how much resistance is needed to get a democratic system that works well. It seems reasonable to me that advanced Condorcet will do, but praxeology can only go so far. If only we had actual experimental data!
(We do have, to some extent. We have Wikimedia elections and Pirate Party primaries. We also know that unless the voters would have reacted to the presence of Condorcet by counter-Condorcet strategy, Condorcet methods would have avoided the IRV crash in Burlington. And if we stretch enough, we have multiwinner ranked voting results, such as with New York, that give some bound on whether they provide multiparty rule.)
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