On Nov 25, 2008, at 11:25 AM, Markus Schulze wrote:
Dear Jonathan Lundell,
Greg Dennis wrote (25 Nov 2008):
I've studied every IRV election for public office
ever held in the United States, most of which have
their full ranking data publicly available, and
every single time IRV elected the Condorcet winner,
something I consider to be a good, though not
perfect, rule of thumb for determining the "right"
winner.
I wrote (25 Nov 2008):
If I remember correctly, Abd wrote that, in every
IRV election for public office ever held in the
USA, the IRV winner was identical to the plurality
winner. Doesn't that mean that -- when we apply
your logic -- plurality voting always elects the
right winner?
You wrote (25 Nov 2008):
Plurality failed in Florida 2000, so we can conclude
that "plurality voting always elects the right winner"
is false.
And when you apply Abd's claim to your conclusion (that
the statement "plurality voting always elects the right
winner" is false), what can you conclude about Greg's
claim?
Greg concludes that IRV, in practice, tends to elect the Condorcet
winner. Does he conclude that it must always be so? I don't think so.
Abd says that the IRV winner in these cases was also the plurality
winner. Again, no claim of necessity.
We might equally well conclude that plurality usually elects the
Condorcet winner, and that it fails infrequently enough that we don't
have examples of IRV correcting a plurality error. (Florida 2000 is an
example of a plurality error that IRV would most likely have corrected.)
My own view is first that we're talking about marginal differences
here, and that PR vs single-winner elections is of much, much greater
interest, and second that the interesting difference between
plurality, IRV and other ranked methods is not in how they count any
particular profile, but rather in how they influence candidate and
voter behavior. In the IRV examples that Greg and Abd adduce, we don't
actually know what the ballots would have looked like if the elections
had used plurality. The set of candidates might well have been
different, the nature of the campaigns different, and voter strategies
different.
Given an IRV election, the question "how would this election have
turned out if plurality had been used" cannot be answered by counting
the IRV first choices.
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