[EM] Possible PO replacement for MMPO
Kristofer Munsterhjelm
km-elmet at munsterhjelm.no
Sat May 31 11:30:40 PDT 2025
While trying to create a four-candidate monotone resistant set method
using geometry, I found a pairwise counting approach that might preserve
(or come closer to preserving) the FBC while avoiding plurality failures.
Call it "favors victor". (A>B)_fv is the number of voters who do not
rank A strictly lower than B. This means that if a voter ranks A and B
equal, that counts as a full point both to (A>B) and (B>A). As such, it
generalizes the tied-at-the-top rule.
I'll leave it to the people with more experience with FBC methods to
test what pairwise methods pass FBC with this counting approach. But it
might be interesting to check minmax, Copeland, and Ranked Robin (since
the tournament Borda count based on fv reduces to Mike Ossipoff's
Summed-Ranks method which he said passes FBC.)
It's possible that it does allow methods to pass FBC but is too
"approval-y" to be of use; that is, that it causes a race to an Approval
bottom because equal-rank is more powerful than strict rank.
In any case, it does get rid of the MMPO "bad-example" problem,
x: A
1: A=C
1: B=C
x: B
where MMPO elects C. Here's a count using fv with x being a billion:
contributes to
A>B A>C B>A B>C C>A C>B
1e9: A>B=C yes yes yes yes
1: A=C>B yes yes yes yes
1: B=C>A yes yes yes yes
1e9: B>A=C yes yes yes yes
(e.g. A>B=C contributes to A>B because A is ranked higher than B, and to
B>C and C>B because B is not lower ranked than C, and C is not lower
ranked than B, respectively). That gives
against
A B C
A --- 1e9+1 2e9+1
for B 1e9+1 --- 2e9+1
C 1e9+2 1e9+2 ---
So the maximum defeat scores (penalties) are
A: 1e9+2
B: 1e9+2
C: 2e9+1
and the minmax outcome is A=B>C, as desired.
-km
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