[EM] IRV is best method meeting 'later no harm'?

[Jonathan Lundell]

On Nov 26, 2009, at 12:52 PM, seppley at alumni.caltech.edu wrote:

> Can it be said that Later No Harm (LNH) is satisfied by the variation of
> IRV that allows candidates to withdraw from contention after the votes are
> cast?

Assuming that the candidates know what the ballots did, then no, it cannot, assuming that the candidates are unconstrained in their withdrawals.

For example, they could agree that all candidates except the one with a plurality of last-choice mentions would withdraw; that certainly fails LNH.

If you constrain the candidates, by rule or assumption, then it depends on the constraints.

> 
> Similar to truly condorcetian methods, Withdrawal//IRV would presumably
> tend to elect candidates who take median positions on the issues.  That
> would create an incentive for candidates who want to win to take median
> positions.
> 
> I'm not claiming median positions are necessarily best.  But I do believe
> there are significant advantages to voting methods that induce candidates
> who want to win to take median positions:  Less polarization and less
> demagoguery.  Stability & incrementalism, since issues (e.g., abortion)
> will tend to get settled until the median position changes significantly. 
> Accountability on more issues, since a candidate who takes a non-median
> position on some issue would be risking that another candidate will take
> the (majority-preferred) median position and match his/her positions on
> other issues, and also since many other issues would already be settled
> (in an accountable way: the median).  Increased competition, since such
> methods would by necessity have very little spoiling (unlike plain IRV). 
> Reduced corruption, since accountability on more issues and increased
> competition would allow voters to rank less corrupt candidates over more
> corrupt candidates.  Reduced cost of campaigning, since "big tent" parties
> (if they continue to exist) would no longer need to conduct primary
> elections to winnow their contenders to one per office.
> 
> * * *
> 
> By the way, if my understanding is correct, IRV is not Single Transferable
> Vote (STV), the single-winner voting method used in Australia & Ireland. 
> IRV severely limits the number of candidates each voter can rank (to 3, if
> my understanding is correct) whereas STV allows (or requires) each voter
> to rank every candidate.  STV satisfies LNH, and many people may consider
> it to be somewhat better than IRV. (STV facilitates greater competition
> and less spoiling, especially if candidates are permitted to withdraw
> after the votes are cast.)
> 
> Since IRV is said to satisfy LNH, then one must say Plurality Rule
> satisfies LNH too, because Plurality Rule can be viewed as just a
> variation of IRV with a smaller limit (one candidate per voter).
> 
> * * *
> 
> If Withdrawal//IRV and Plurality Rule satisfy LNH, then I think it's
> reasonable to say that so do some methods in the Voting for a%2
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