[EM] IRV vs Condorcet - a challenge

[Jan Kok]

On 6/13/06, Dave Ketchum <davek at clarityconnect.com> wrote:
> On Mon, 12 Jun 2006 23:36:45 -0500 Dan Bishop wrote:
>
> > Dave Ketchum wrote:
> >
> >>      Permitting equal ranking for multiple candidates.  Desirable for
> >>pleasing those who call Approval desirable; doable with Condorcet, though
> >>there can be arguments as to counting' Not doable as practical for IRV
> >>(explain how if you disagree).
> >>
> >
> > Some simple ways of dealing with equal rankings in IRV/STV are:
> >
> > (1) "Transferrable Cumulative Vote" - If there are N top-ranked
> > non-eliminated candidates on a ballot, then each candidate receives 1/N
> > of a vote.  (This is what I used in my STV-CLE calculator.)
>
>
> Looks fair - weakens the best liked candidate while giving the others extra.

The motivation for equal-ranking was to allow people to vote
Approval-style. I don't think anyone is clamoring to vote
Cumulative-style... Another motivation for equal-ranking is to allow
voters to rank a large number of candidates on a ballot which allows
just a few ranks.

Like simple Cumulative Voting, with TCV there is an incentive to "go
for broke", to put all of one's votes on one candidate. Given a ballot
that allows three ranks, alternative party supporters might vote for
just their favorite as first choice (rather than sharing first
choice), in order to try to keep their favorite from getting
eliminated early. Then for second choice, vote Plurality-style for the
front-runner among the acceptable candidates (e.g. Nader or Perot or
the Unity08.org candidate if they get anywhere). Then vote for the
lesser-evil of the D and R candidates as third choice.

The Cumulative aspect of TCV pressures such voters to omit some of the
candidates they like better than their voted second choice.

> > (2) "Transferrable Approval Vote" - Each top-ranked non-eliminated
> > candidate on a ballot receives 1 vote.
>
>
> NOT fair - gives each such candidate an extra boost.

I don't think I have ever seen "fair" defined in a way that is
mathematically precise _and_ non-controversial.

Anyway, this seems no more or less fair than plain Approval Voting.

In the above scenario, one could vote for one's favorite as first
choice, all of one's "acceptable" candidates as 2nd choice, and the
lesser evil as 3rd choice.

In a close Left-Center-Right race where a widely acceptable Center
might get squeezed out, Left and Right supporters can equal-rank their
favorite and Center in first place. But I would rather that candidates
be eliminated by comparing the two with the least first-choices and
eliminating the one who is less preferred on the majority of all
ballots. I.e. use BTR-IRV, not IRV. That results in a Condorcet
method, and reduces the frequency of having to make the difficult
decision whether to equal-rank Center with your favorite.

> > (3) Convert every ballot into a fully-ranked one by breaking ties at random.
>
>
> Looks fair.

I think that's roughly equivalent to the 1st method (TCV) proposed above.

- Jan

>
> --
>   davek at clarityconnect.com    people.clarityconnect.com/webpages3/davek
>   Dave Ketchum   108 Halstead Ave, Owego, NY  13827-1708   607-687-5026
>             Do to no one what you would not want done to you.
>                   If you want peace, work for justice.
>
>
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