[EM] Historical perspective about FairVote organization
km_elmet at lavabit.com
Fri Mar 15 02:12:05 PDT 2013
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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