[EM] Ordering defeats in Minimax
stepjak at yahoo.fr
Tue Apr 25 06:34:28 PDT 2017
Yes, WV satisfies Plurality. In fact margins methods are among the only serious proposals that don't satisfy it. This propertysays that e.g. A can't win in this scenario:
7 A>B5 B8 C
Methods that have added Smith compliance to MinMax usually call for WV. Schulze's paper describing the Schulze methodused to say that there had been discussion concluding that margins wasn't recommended due to strategic reasons, though Ican't find that language there anymore. That being said, I would be a bit surprised to learn that anyone using Schulze has chosen a margins version of it.
The issue is pretty controversial and becomes complicated when you get into the strategic implications. Arguments that margins just "makes more sense" than WV are common, but I find them rather abstract in comparison to what I think voterswould expect from a method.
As an example of the latter, suppose there is only one pairwise contest A>B in which all of the voters expressed a strictpreference. Say it was very close, 51%-49%. The margins logic is that this contest is fair game to be discarded because ofhow close a race it was. But I would say that this contest is probably the one that the voters view as the most important one,everyone having voted on it, and they would not expect its loser to be elected. They (at least the 51%) will likely view thisas a spoiler situation, as whatever other candidates were in the race (to cause this outcome) did not themselves win; theysimply threw the race to the candidate who (without their presence) would have lost.
The desire to say that a 35-0 win is stronger than a 51-49 win could make sense if a strong win in itself was of some value.But all that realistically matters is who wins the entire race. If the 35-0 winner is not in a position to get elected, there is nothing to be gained in letting that contest affect the outcome. It doesn't aid him or his supporters, it is pure noise. It couldas well be treated as zero strength.
De : Kristofer Munsterhjelm <km_elmet at t-online.de>
À : Andrew Myers <andru at cs.cornell.edu>; Election Methods <election-methods at lists.electorama.com>
Envoyé le : Mardi 25 avril 2017 3h08
Objet : Re: [EM] Ordering defeats in Minimax
On 04/25/2017 05:36 AM, Andrew Myers wrote:
> Hi all,
> I recently added Minimax to the algorithms supported by CIVS, because it
> has some nice properties, especially regarding stability of the
> ordering. However, there's a bit of a challenge lurking. Minimax as
> classically defined assumes that all ballots are totally ordered. CIVS
> allows ties, however. So for any pair of alternatives there is a (W, L)
> pair where W is the number of people who prefer the first alternative
> and L is the number who prefer the second.
> Recall that Minimax chooses the alternative whose strongest defeat is
> the weakest. What is the right way to define the ordering on defeats?
> 1. WV: (W1, L1) > (W2, L2) if W1 > W2 or (W1=W2 and L2 > L1)
> [currently implemented]
> 2. Margins: (W1, L1) > (W2, L2) if W1 - L1 > W2 - L2
> 3. LV: (W1, L1) > (W2, L2) if L1 < L2 or (L1 = L2 and W1 > W2)
> I'm sure this has been discussed already at great length. Your advice is
It depends on what properties you want. The most obvious difference that
comes to mind: Minmax(wv) passes the Plurality criterion (if I recall
correctly) whereas Minmax(margins) passes Mono-add-top; but wv doesn't
pass MAT and margins doesn't pass Plurality.
I'm usually a wv person, but I think Minmax is more classically
associated with margins. Or perhaps I think that because Juho is here
and he prefers margins :-)
On a side note, Minmax can produce a lot of ties if there are few voters
involved, so sometimes I prefer to break ties by second strongest defeat
(and then third strongest, fourth strongest, etc). That isn't
*classical* Minmax, but it shouldn't break any of Minmax's criteria.
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