[EM] Why I Prefer IRV to Condorcet

[Kevin Venzke]

Hi Greg,

--- En date de : Mer 19.11.08, Greg <greg at somervilleirv.org> a écrit :
> De: Greg <greg at somervilleirv.org>
> Objet: [EM] Why I Prefer IRV to Condorcet
> À: election-methods at electorama.com
> Date: Mercredi 19 Novembre 2008, 15h28
> I have written up my reasons for preferring IRV over
> Condorcet methods
> in an essay, the current draft of which is available here:
>   http://www.gregdennis.com/voting/irv_vs_condorcet.html
> 
> I welcome any comments you have.
> 
> Thanks,
> Greg

I want to comment on the first point/reason. I'll quote from the page.

>First and foremost, IRV eliminates the most common type of Condorcet 
>failure --- the "spoiler" scenario --- where the presence of a candidate 
>with little core support causes a Condorcet winner with strong core 
>support to lose.

I don't understand what kind of scenario you're referring to. I thought I
did, and was going to say that good Condorcet methods don't behave in
that way. But then I noticed the term "core support," which puzzles me
in the context of Condorcet spoilers.

>Admittedly, there is another situation similar to the spoiler problem --- 
>the "center squeeze" scenario -- in which IRV may fail to elect the 
>Condorcet winner. In this scenario, the presence of a candidate with 
>strong core support causes a Condorcet winner with little core support to 
>lose. Fortunately, despite the theoretical possibility of this scenario, 
>the empirical evidence suggests that it is vanishingly rare in practice. 
>Despite the hundreds of public IRV elections that are conducted worldwide 
>every year, the actual concrete examples of it occurring in practice are 
>few and far between.

A problem with using IRV elections to judge whether IRV suffers from
a center squeeze effect, is that it overlooks the possibility that IRV's
nomination incentives deter would-be Condorcet winners from running
(due to the fact that everyone knows they would not win).

You can make the argument that plurality also has very good Condorcet
efficiency since it is never observed to fail to elect a Condorcet winner.
Even adding the ability to rank lower preferences would probably not
change this, since with no real change to the method there is also no
real change to the nomination incentives.

>Lacking sufficient examples of real elections in which IRV has failed to 
>elect the Condorcet winner, a few IRV critics have resorted to using top-
>two runoff elections in which the Condorcet winner lost as evidence of 
>IRV's center-squeeze problem. However, top-two runoff and instant runoff 
>are different systems that can produce different results, so 
>this "evidence" is hardly convincing.

Actually some of us will argue that top-two runoff seems likely to have
better Condorcet efficiency (in the abstract sense) than IRV. I can see
an argument for both sides. But I would agree that they are different 
systems with different incentives.


I have a few problems with #4... Partly that I find the arguments
speculative wrt candidate behavior, and partly that I don't see an 
inherent advantage in knowing where candidates stand if everything is 
still going to come down to competing blocs of core support that probably
dislike each other. Mostly, and related, it's that if we agree that we 
can't trust voters to give us meaningful lower preferences, then I lose 
most of my enthuasism for voting methods. If we can only trust first 
preferences, and candidates that get a lot of first preferences, how much 
room do we really have to make improvements? We can stray from plurality
hardly at all.

Kevin Venzke