Best-of-Both-Worlds Fairness in Committee Voting
Abstract
The best-of-both-worlds paradigm advocates an approach that achieves desirable properties both ex-ante and ex-post. We launch a best-of-both-worlds fairness perspective for the important social choice setting of approval-based committee voting. To this end, we initiate work on ex-ante proportional representation properties in this domain and formalize a hierarchy of notions including Individual Fair Share (IFS), Unanimous Fair Share (UFS), Group Fair Share (GFS), and their stronger variants. We establish their compatibility with well-studied ex-post concepts such as extended justified representation (EJR) and fully justified representation (FJR). Our first main result is a polynomial-time algorithm that simultaneously satisfies ex-post EJR, ex-ante GFS and ex-ante Strong UFS. Subsequently, we strengthen our ex-post guarantee to FJR and present an algorithm that outputs a lottery which is ex-post FJR and ex-ante Strong UFS, but does not run in polynomial time.
Keywords
Cite
@article{arxiv.2303.03642,
title = {Best-of-Both-Worlds Fairness in Committee Voting},
author = {Haris Aziz and Xinhang Lu and Mashbat Suzuki and Jeremy Vollen and Toby Walsh},
journal= {arXiv preprint arXiv:2303.03642},
year = {2023}
}
Comments
Appears in the 19th Conference on Web and Internet Economics (WINE), 2023