[EM] Why I Prefer IRV to Condorcet

[Greg]

Thanks, Chris. I'll correct the errors and rephrase some things I
didn't say correctly.

On the Compromise strategy, I think some compromises are more
intuitive than others. I think it's intuitive to abandon a more weakly
supported candidate, e.g. Nader, in favor of a major candidate, as is
common in FPTP. But it strikes me as more counter-intuitive, at least
for the average voter, to abandon a candidate with strong core support
in favor of a more weakly supported candidate, as could happen under
IRV. Then there's the issue as to whether the result of the
strategizing is a better or worse result overall . . . but that's a
tricky topic for another time.


> Date: Thu, 20 Nov 2008 11:51:01 -0800 (PST)
> From: Chris Benham <cbenhamau at yahoo.com.au>
> Subject: [EM]  Why I Prefer IRV to Condorcet
>
> Greg,
> I generally liked your essay. I rate IRV as the best of the single-winner methods that
> meet Later-no-Harm, and a good method (and a vast improvement on FPP).
>
> But I think you made a couple of technical errors.
>
> "However, because bullet voting can help and never backfire against one's top choice under
> Condorcet, expect every campaign with a shot at winning to encourage its supporters to
> bullet vote. "
>
> Bullet voting can "backfire against one's top choice under Condorcet" because Condorcet
> methods, unlike IRV, fail Later-no-Help.
>
> http://groups.yahoo.com/group/election-methods-list/files/wood1996.pdf
>
> In this 1996 Douglas Woodall paper, see "Election 6" and the accompanying discussion on
> page 5/6 of the pdf (labelled on the paper as "Page 13").
>
> Quoting again from your paper:
> "As mentioned, every voting system is theoretically vulnerable to strategic manipulation, and IRV
> is no exception. However, under IRV, there is no strategy that can increase the likelihood of
> electing one's first choice beyond the opportunity offered by honest rankings. While there are
> strategies for increasing the chances of less preferred candidates under IRV, like push-over,
> they are counter-intuitive."
>
> The Push-over strategy is certainly not limited to improving the chance of electing a "lower
> [than first] choice". Say sincere is:
>
> 49: A?
> 27: B>A
> 24: C>B
>
> B is the IRV winner, but if? 4-21 (inclusive) of the A voters change to C or C>? then the winner
> changes to A.
>
> But as you say the strategy isn't "intuitive" , and backfires if too many of the A supporters try it.
> Some IRV opponents claim to like Top-Two Runoff, but that is more vulnerable to Push-over
> than IRV (because the strategists can support their sincere favourite in the second round).
>
> The quite intuitive strategy that IRV is vulnerable to is Compromise, like any other method that
> meets Majority. But voters' incentive to compromise (vote one's front-runner lesser-evil in first
> place to reduce the chance of front-runner greater-evil winning) is generally vastly vastly less
> than it is under FPP.
>
> (There are methods that meet both Majority and Favourite Betrayal, and in them compromisers
> can harmlessly vote their sincere favourites in equal-first place.)
>
> But some Condorcet advocates are galled? by the Compromise incentive that can exist where
> there is a sincere CW who is not also a sincere Mutual Dominant Third winner.
>
> 49: A>B
> 02: B>A
> 22: B
> 27: C>B
>
> On these votes B is the CW, but IRV elects A.? If the C>B voters change to B then B will be
> the voted majority favourite, so of course IRV like Condorcet methods and FPP will elect B.
>
> Chris Benham