# Ranking from Stochastic Pairwise Preferences: Recovering Condorcet Winners and Tournament Solution Sets at the Top

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Arun Rajkumar,
Suprovat Ghoshal,
Lek-Heng Lim,
Shivani Agarwal
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Proceedings of the 32nd International Conference on Machine Learning, PMLR 37:665-673, 2015.

#### Abstract

We consider the problem of ranking n items from stochastically sampled pairwise preferences. It was shown recently that when the underlying pairwise preferences are acyclic, several algorithms including the Rank Centrality algorithm, the Matrix Borda algorithm, and the SVM-RankAggregation algorithm succeed in recovering a ranking that minimizes a global pairwise disagreement error (Rajkumar and Agarwal, 2014). In this paper, we consider settings where pairwise preferences can contain cycles. In such settings, one may still like to be able to recover ‘good’ items at the top of the ranking. For example, if a Condorcet winner exists that beats every other item, it is natural to ask that this be ranked at the top. More generally, several tournament solution concepts such as the top cycle, Copeland set, Markov set and others have been proposed in the social choice literature for choosing a set of winners in the presence of cycles. We show that existing algorithms can fail to perform well in terms of ranking Condorcet winners and various natural tournament solution sets at the top. We then give alternative ranking algorithms that provably rank Condorcet winners, top cycles, and other tournament solution sets of interest at the top. In all cases, we give finite sample complexity bounds for our algorithms to recover such winners. As a by-product of our analysis, we also obtain an improved sample complexity bound for the Rank Centrality algorithm to recover an optimal ranking under a Bradley-Terry-Luce (BTL) condition, which answers an open question of Rajkumar and Agarwal (2014).

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