[EM] Ranked Pairs

Colin Champion colin.champion at routemaster.app
Sun Sep 17 01:25:04 PDT 2023

Michael - I don't think I understand everything you're saying. I was 
referring to RP (margins), which I believe is the form in which Tideman 
specified it, and I assumed that truncation was a mixture of laziness 
and ignorance. (Actually the simulation I referred to assumed mandatory 
truncation, which I think is common in the US.) I don't hold any brief 
for RP; on the contrary I much prefer minimax (also margins).
    I do indeed think that voters will have difficulty in rank-ordering 
lots of candidates, or even in examining and choosing between them. I 
hoped that the primary I proposed would make the best of voters who were 
unable to assess the entire field, since if they voted for someone 
they'd heard of, and that candidate got eliminated, then their vote 
would be transferred to someone they might like, even if they didn't 
realise it.

On 17/09/2023 06:42, Michael Ossipoff 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.
> 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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