[EM] Fwd: Ranked Pairs

Michael Ossipoff email9648742 at gmail.com
Wed Sep 20 14:11:51 PDT 2023

Thanks for the experiment. I hope that Tideman’s organization won’t be
promoting the old margins version of RP.  …unless just for maybe choosing
between pizza-toppings or movies.

On Wed, Sep 20, 2023 at 07:30 Colin Champion <colin.champion at routemaster.app>

> Well, I coded up Minimax(WV) for my own evaluation. Rather to my disgust
> (and contrary to Darlington) I find that it does indeed outperform Margins
> for truncated sincere ballots. I ran a large number of trials with 10001
> voters under a spatial model, 8 candidates being truncated to 4. Minimax
> (margins)=83.35% correct, minimax(wv)=84.09%. Other methods which
> outperformed standard minimax in the simulation include Approval Sorted
> Margins (in an ordinal version suggested by Ted Stern): 84.14%; Black:
> 84.22%; Smith,Borda: 84.22%.
>    Hastily written and unreliable code, not to be trusted.
>       CJC
> On 18/09/2023 22:30, Michael Ossipoff wrote:
> 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> wrote:
>> 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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