Committee Monotonic Proportional Representation: A New Voting Rule and Impossibility Results

H Aziz, P Lederer, A Ritossa - arXiv preprint arXiv:2406.19689, 2024 - arxiv.org
arXiv preprint arXiv:2406.19689, 2024arxiv.org
We study committee voting rules under ranked preferences, which map the voters'
preference relations to a subset of the alternatives of predefined size. In this setting, the
compatibility between proportional representation and committee monotonicity is a
fundamental open problem that has been mentioned in several works. We design a new
multi-winner voting rule called the Solid Coalition Refinement (SCR) Rule that
simultaneously satisfies committee monotonicity and Dummett's PSC as well as one of its …
We study committee voting rules under ranked preferences, which map the voters' preference relations to a subset of the alternatives of predefined size. In this setting, the compatibility between proportional representation and committee monotonicity is a fundamental open problem that has been mentioned in several works. We design a new multi-winner voting rule called the Solid Coalition Refinement (SCR) Rule that simultaneously satisfies committee monotonicity and Dummett's PSC as well as one of its variants called inclusion PSC. This is the first rule known to satisfy both of these properties. Moreover, we show that this is effectively the best that we can hope for as other fairness notions adapted from approval voting such as Rank-JR and Rank-PJR+ are incompatible with committee monotonicity.
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