Extensions of the Simpson voting rule to the committee selection setting

Daniela Bubboloni (1) , Mostapha Diss (2) , Michele Gori (1)

1 UniFI - Università degli Studi di Firenze = University of Florence = Université de Florence
2 GATE Lyon Saint-Étienne - Groupe d'Analyse et de Théorie Economique Lyon - Saint-Etienne

Committee selection rules are procedures selecting sets of candidates of a given size on the basis of the preferences of the voters. There are in the literature two natural extensions of the well-known single-winner Simpson voting rule to the multiwinner setting. The first method gives a ranking of candidates according to their minimum number of wins against the other candidates. Then, if a fixed number k of candidates are to be elected, the k best ranked candidates are chosen as the overall winners. The second method gives a ranking of committees according to the minimum number of wins of committee members against committee nonmembers. Accordingly, the committee of size k with the highest score is chosen as the winner. We propose an in-depth analysis of those committee selection rules, assessing and comparing them with respect to several desirable properties among which unanimity, fixed majority, non-imposition, stability, local stability, Condorcet consistency, some kinds of monotonicity, resolvability and consensus committee. We also investigate the probability that the two methods are resolute and suffer the reversal bias, the Condorcet loser paradox and the leaving member paradox. We compare the results obtained with the ones related to further well-known committee selection rules. The probability assumption on which our results are based is the widely used Impartial Anonymous Culture.

Mots clés: Multiwinner Elections; Committee Selection Rule; Simpson Voting Rule; Paradoxes; Probability

Domaines: Economies et finances

Daniela Bubboloni, Mostapha Diss, Michele Gori. Extensions of the Simpson voting rule to the committee selection setting. 2018. ⟨halshs-01827668⟩