An algorithmic view of voting

Abstract

We offer a novel classification of voting methods popular in social choice theory. Our classification is based on the more general problem of rank aggregation in which, beyond electing a winner, we also seek to compute an aggregate ranking of all the candidates; moreover, our classification is offered from a computational perspective-based on whether or not the voting method generalizes to an aggregation algorithm guaranteed to produce solutions that are near optimal in minimizing the distance of the aggregate ranking to the voters' rankings with respect to one of three well-known distance measures: the Kendall tau, the Spearman footrule, and the Spearman rho measures. We show that methods based on the average rank of the candidates (Borda counting), on the median rank of the candidates, and on the number of pairwise-majority wins (Copeland) all satisfy the near-optimality criterion with respect to each of these distance measures. On the other hand, we show that natural extensions of each of plurality voting, single transferable voting, and Simpson-Kramer minmax voting do not satisfy the near-optimality criterion with respect to these distance measures.