[EM] RCIPE version 2

[Kristofer Munsterhjelm]

On 21.07.2021 03:19, VoteFair wrote:
> Version 2 of the Ranked Choice Including Pairwise Elimination (RCIPE)
> voting method is now coded.
> 
> Here's a link to the scatter plot that shows its Clone Independence and
> IIA (Independence of Irrelevant Alternatives) success rates compared to
> other single-winner methods:
> 
>   https://www.rankedchoiceoregon.org/img/clone_iia_success_rates.jpg
> 
> Based on Kristofer's feedback on the scatter plot, I've moved the IRV
> (instant-runoff voting) data points to the top to show that IRV has a
> zero Clone Independence failure rate.  (The CI measurements yield a
> non-zero CI failure rate because IRV does not specify a tie breaker
> method and there are lots of lower-level ties in these "stress test"
> measurements.)  I've removed the IRV-BTR data because it's so similar to
> IRV.
> 
> Here is how RCIPE version 2 is counted:
> 
> * Eliminate the "pairwise losing candidate" (Condorcet loser) when there
> is one.
> 
> * Calculate a "pairwise support count" for each candidate.  Each ballot
> contributes to that count the number of remaining(!) candidates who are
> ranked lower than the specified candidate.

I would of course suggest moving the tiebreaker up here to replace the
current elimination condition :-)

"Eliminate the candidate who has the smallest one-on-one pairwise count
against any other remaining candidate."

You need the pairwise matrix to determine the Condorcet loser anyway, so
there's not much of a gain in complexity.

-km