[EM] Fwd: Ranked Pairs
Colin Champion
colin.champion at routemaster.app
Mon Sep 18 07:02:38 PDT 2023
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
> <mailto: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
> <mailto: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 <mailto:forest.simmons21 at gmail.com>> wrote:
>
>
>
> On Sat, Sep 16, 2023, 9:42 PM Michael Ossipoff
> <email9648742 at gmail.com <mailto: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
> <mailto: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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