[EM] Revised: Instant Pairwise Elimination (IPE)
cbenham at adam.com.au
Mon Jan 13 07:56:30 PST 2020
I missed the earlier discussion on this.
So what if an elimination round /does /have a "pairwise-losing
candidate", what then?
And what exactly is a "pairwise-losing candidate"?
On 13/01/2020 12:14 pm, VoteFair wrote:
> Based on a suggestion from a user on Reddit, I have revised the
> definition of the Instant Pairwise Elimination method that previously
> I published at Democracy Chronicles and then discussed here.
> The method still successively eliminates pairwise (Condorcet) losers.
> Now, instead of resolving Condorcet (rock-paper-scissors) cycles using
> an "upside-down" version of instant-runoff voting (IRV), it uses
> pairwise counts as described here:
> "If an elimination round has no pairwise-losing candidate, then the
> method eliminates the candidate with the largest pairwise opposition
> count, which is determined by counting on each ballot the number of
> not-yet-eliminated candidates who are ranked above that candidate, and
> adding those numbers across all the ballots. If there is a tie for the
> largest pairwise opposition count, the method eliminates the candidate
> with the smallest pairwise support count, which similarly counts
> support rather than opposition. If there is also a tie for the
> smallest pairwise support count, then those candidates are tied and
> all those tied candidates are eliminated in the same elimination round."
> Below are my guesses for which fairness criteria it fails and passes.
> Please tell me which guesses are not correct.
> Condorcet: fail
> Condorcet loser: pass
> Ranks equal: pass
> Ranks greater than 2: pass
> Polytime: pass
> Resolvable: pass
> Majority: fail
> Majority loser: fail
> Mutual majority: fail
> Smith/ISDA: fail
> LIIA: fail
> IIA: fail
> Cloneproof: fail
> Monotone: fail
> Consistency: fail
> Reversal symmetry: fail
> Later no harm: fail
> Later no help: fail
> Burying: fail
> Participation: fail ?
> No favorite betrayal: fail ?
> Summable: O(N!) ?
> As I've said many times, it's the frequency with which the failures
> occur that is much, much more important than simply counting how many
> criteria it fails. I suspect that its frequencies of failure will be
> quite low compared to most other single-winner methods, and may
> approach the low frequencies that I believe characterize the
> Condorcet-Kemeny method.
> I've created a page for this method on Electowiki. You are welcome to
> edit that page with any corrections.
> BTW, I realize that it's possible that the alternate elimination
> method always identifies the pairwise/Condorcet loser (if there is
> one). If so, this would mean that the description could be
> "simplified" to a single step (actually two steps in case there is a
> tie). However, for the benefit of most voters who are not comfortable
> with mathematics it's important to explicitly state that the first
> priority is to eliminate the pairwise loser.
> Of course software that implements the method would do the
> calculations using a much faster method than the counting method
> described above. The description above is written to be understandable
> to people who are not already familiar with pairwise counting.
> In advance, thank you for any feedback.
> Richard Fobes
> Election-Methods mailing list - see https://electorama.com/em for list
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