[EM] Simple Acceptable Ranked Choice Voting

Kevin Venzke stepjak at yahoo.fr
Tue Jan 17 04:25:26 PST 2023

Hi Forest,

> Kevin,
> The funny thing is that in the Round Robin sports tournament context the MMPS method
> for picking the winning team seems to be monotone ... one team can get more points against
> another without affecting any other pairwise scores ... in the pairwise matrix only one
> entry changes.
> Maybe we could call that Tournament Monotonicity. 

MMPS alone is monotone. It's the chain-building process to find an uncovered winner which
will break it.

> How about the Chicken Defense and MMPS?

This question turned out to be complicated. Two really big issues here.

1. The CD criterion seems to suppose that if the fragmented majority were to cooperate, then
they *would* win, and be rewarded for not defecting from each other. However, MMPS doesn't
satisfy mutual majority, so the reward can't be promised.

(Is FPP a "good CD method"? I think it's not in the spirit of it at all.)

This should mean that there is even more incentive to just use favorite betrayal (or have
one candidate drop out). But in MMPS we don't have to use full order reversal as we have
equal ranking and weak FBC. So that leads to the next issue.

2. Can a method that allows equal ranking (and allows it to do something useful, as with a
weak FBC promise) be considered to satisfy "CD" at all? This creates a separate, likely way
for the fragmented majority to win, where many voters still defect. That really seems to
undermine the mechanism that's supposed to be working here.

That is, either faction can reason "We can defect, and force the other faction to use equal
ranking to save the win and elect our guy."


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