[EM] Fwd: Ranked Pairs

Michael Ossipoff email9648742 at gmail.com
Mon Sep 18 13:30:02 PDT 2023

Hi Colin—

Yes, margins beats wv for social-utility under sincere voting when there’s
that vanishingly rare natural top-cycle.

But Darlington & Tideman evidently aren’t considering resistance to
offensive strategy, which is a much bigger threat than natural top-cycles.

Protecting the CW from offensive strategy is more important than SU in
natural top-cycles.

…& is better for SU.

On Mon, Sep 18, 2023 at 07:03 Colin Champion <colin.champion at routemaster.app>

> Thanks to Kevin and Michael for pointing out a feature of minimax I was
> unaware of. I had however seen Richard Darlington's paper [1] in which he
> referred to 'several studies' comparing margins with winning votes. He
> reports that margins 'was the big winner in all of them'. I suppose I'll
> have to look deeper.
>    Colin
> [1]. https://arxiv.org/abs/1807.01366
> On 18/09/2023 07:57, Michael Ossipoff wrote:
> ---------- Forwarded message ---------
> From: Michael Ossipoff <email9648742 at gmail.com>
> Date: Sun, Sep 17, 2023 at 22:54
> Subject: Re: [EM] Ranked Pairs
> To: Forest Simmons <forest.simmons21 at gmail.com>
> This was meant to be sent by “Reply All”, in order to post it. So now I’m
> forwarding it to EM.
> Forest—
> But wv prevents truncation (strategic or otherwise) from taking the win
> from a CW.
> …&, with, wv, refusing to rank anymore you don’t approve will cause
> offensive order-reversal by their preferrers to backfire.
> I’d always take that precaution, & would advise others to.
> When we discussed these guarantees years ago they seemed absolute, & we
> still have the guarantee-criteria based on them…met by wv versions of
> MinMax, RP, CSSD, & Smith//MinMax.
> …&, with MinMax, whose winner can come from anywhere, not just from the
> top-cycle, & so, offensive order-reversal, when there are a fair number of
> candidates, is unpredictable & risky for its perpetrators, even if the
> precaution of deterrent-truncation isn’t taken.
> On Sun, Sep 17, 2023 at 21:17 Forest Simmons <forest.simmons21 at gmail.com>
> wrote:
>> On Sat, Sep 16, 2023, 9:42 PM Michael Ossipoff <email9648742 at gmail.com>
>> wrote:
>>> Is that RP(wv), or RP(margins) ?
>>> RP(wv) would thwart & deter offensive strategy, an important property in
>>> public elections.
>>> …&, actually, it seems to me that MinMax(wv) would do that better.
>>> That’s because, choosing only from the Smith Set RP, limits it’s choice
>>> to the strategic top-cycle that created by the offensive strategists.
>>> Suppose that the CW’s preferrers don’t do defensive truncation (never
>>> rank anyone you wouldn’t approve in Approval, or whose preferrers you
>>> regard as likely to offensively order-reverse) ?
>>> Knowing that RP will limit its choice to their small  strategic
>>> top-cycle, it would be easier for the strategists to be fairly sur that
>>> their candidate would win in that top-cycle.
>>> But, with MinMax, the winner is chosen more broadly, & could be anywhere
>>> in the candidate-set.  …making it more difficult & risky to confidently do
>>> offensive order/reversal.
>>> RP(margins) might the best choice for a completely honest electorate,
>>> but MinMax(wv) seems better for public elections, due to its better
>>> thwarting & deterrence of offensive strategy.
>>> Yes, MinMax doesn’t meet the luxury cosmetic look-good criteria that RP
>>> meets.
>>> But for one thing, I remind you that natural ( sincere) top-cycles are
>>> vanishingly-rare.
>> This is the same conclusión I have come around to.
>> And methods that break a three member top cycle at the weakest link tend
>> to reward the burier faction.
>>> So do you want to have less strategy-protection, in order for the result
>>> to maybe look better in a vanishingly rare natural top/cycle?
>>> …& how bad is a violation of Condorcet-Loser anyway.  “Beaten by all the
>>> other alternatives” sounds like some kind of unanimity, but of course it
>>> isn’t. It isn’t like a Pareto-violation. I remind you that the MinMax
>>> winner has fewer voters preferring some particular candidate over him than
>>> anyone else does.
>>> Clone-Criterion violation? How bad that really in MinMax, especially
>>> when we’re talking about a vanishingly rare natural top-cycle?
>>> RP(margins) for a completely honest electorate.
>>> MinMax(wv) for public elections.
>>> ..& about a primary to reduce the candidates to 5: Forget the primary.
>>> If you think people will have trouble rank-ordering lots of candidates, I
>>> remind you that, to vote among them in a primary, they’d still have to
>>> examine & choose among the initial many candidates.
>>> …harder than ranking only the ones you know & regard as deserving &
>>> definitely in your accepts& preferred set.
>>> On Wed, Sep 13, 2023 at 00:18 Colin Champion <
>>> colin.champion at routemaster.app> wrote:
>>>> I notice that RP is the only election method mentioned by name in the
>>>> Virginia agenda.
>>>> A while ago I ran some simulations on elections with truncated ballots.
>>>> Something I noticed was that the presence of RP in the list of methods
>>>> made the software unacceptably slow. I didn't look into the cause, but
>>>> there's a natural explanation, which is the fact that RP is known to be
>>>> NP-complete when it deals correctly with tied margins, i.e. by
>>>> exhausting over all their permutations. Presumably if some candidates
>>>> are unpopular and ballots are extensively truncated, then tied margins
>>>> are much likelier than with complete ballots.
>>>> I gather that practical implementations of RP choose a random
>>>> permutation rather than exhausting. This seems to me to bring a danger.
>>>> The presence of a few vanity candidates (truncated off almost all
>>>> ballots) may lead to ties, and this may lead to a comfortable winner
>>>> looking as though he owes his victory to a coin-toss. Obviously this
>>>> undermines the legitimacy of his win.
>>>> CJC
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