[EM] Why I Prefer IRV to Condorcet

[Juho Laatu]

Here's one IRV example with three strong candidates and where voters do have some incentive to compromise.

45: A>B>C
10: B>A>C
15: B>C>A
30: C>B>A

We have one centrist candidate (B) between two others.

According to this poll it seems that B will be eliminated first, and then A would win since some B supporters prefer A to C.

If sufficient number of C supporters would abandon their favourite and vote B>C>A, then C would be eliminated first and the centrist candidate B would be elected.

Based on this poll it seems that if C voters don't compromise (or if C will not withdraw) then from C supporters' point of view the worst candidate (A) will be elected.

- This situation could be reasonably common (or plausible) in real life
- B is a Condorcet winner ((that IRV would not elect))
- B seems to be politically closer to C than to A
- C is not a weak candidate since with few more "core" voters or second place support it could beat A (if the strong centrist candidate B will be eliminated first)

C supporters could be optimistic and hope for a change in opinions before the election day. I mean that in real elections many voters may be optimistic and fighting spirited and believe rather in those earlier polls that gave their favourite more votes than this poll etc.

The strategy of the C voters is not very "intuitive" in the sense that it is never natural to abandon one's favourite (it could be easier e.g. to rank the strongest competitor last even if that would be an irrational strategy). But on the other hand it is quite straight forward to see from the poll results (maybe voiced out by media) that indeed it makes sense for the C supporters to give up and abandon C if people will vote as indicated in this poll. The voters will thus have a dilemma, whether to vote sincerely or whether to compromise.

Juho



--- On Sat, 22/11/08, Greg <greg at somervilleirv.org> wrote:

> From: Greg <greg at somervilleirv.org>
> Subject: Re: [EM] Why I Prefer IRV to Condorcet
> To: election-methods at lists.electorama.com
> Date: Saturday, 22 November, 2008, 3:06 AM
> 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
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