[EM] Why I Prefer IRV to Condorcet

[Juho Laatu]

--- On Sun, 23/11/08, Kristofer Munsterhjelm <km-elmet at broadpark.no> wrote:

> Regarding number two, simple Condorcet methods exist.
> Borda-elimination (Nanson or Raynaud) is Condorcet. Minmax
> is quite simple, and everybody who's dealt with sports
> knows Copeland (with Minmax tiebreaks). I'll partially
> grant this, though, since the good methods are complex

Minmax is both simple and good. I think at least minmax(margins) is a good solution for many needs.

The weakest spot of minmax(margins) could be that it fails mutual majority. That means for example that nominating a set of clones instead of just one candidate could lead (at least in theory) to not winning the election.

On the other hand other methods than minmax(margins) may not respect the good idea of mmm to elect the candidate that has the least incentive among voters to be changed to some other of the candidates.

(Minmax(margins) fails also Smith and Condorcet loser, but those violations can be explained to be intentional and positive.)

> , but
> I'll ask whether you think MAM (Ranked Pairs(wv)) is too
> complex. In MAM, you take all the pairwise contests, sort by
> strength, and affirm down the list unless you would
> contradict an earlier affirmed contest. This method is
> cloneproof, monotonic, etc...
> 
> Perhaps you could explain it in that "say A won.
> B's supporters are going to say "but some people
> preferred B to A!". Then you can say, but more people
> preferred C to B and A to C". I'm not sure, there
> may be better explanations.

Also minmax(margins) is close to this. It has a very natural explanation. (I gave one rough explanation above. Another one is "elect the candidate that needs least additional votes to win all others".)


I don't claim that Minmax(margins) would be the best Condorcet method for all needs. I rather claim that there are many kind of elections and there are many alternative targets. Minmax (margins) emphasizes small opposition (in favour of any other single candidate) against the elected candidate.

This justification focuses on the performance with sincere votes. Also other good criteria that describe which candidate would be the best may be used..

Another direction is to look for a method that is most resistent to straegic voting. (Many of the best known criteria emphasize this viewpoint.)

If the environment where the method will be used in plagued with widespread strategic voting then it makes sense to emphasize the "strategy free" oriented criteria a bit. If the voters are expected to be predominantly sincere then one has the luxury to focus on criteria that aim at electing the best winner.

There are thus different kind of environments and different kind of needs. One should pick the best method for each need and environment. Somewhere it may be e.g. FPTP or minmax(margins), somewhere something else.

Juho