Learning Mixtures of Plackett-Luce Models from Structured Partial Orders
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Mixtures of ranking models have been widely used for heterogeneous preferences. However, learning a mixture model is highly nontrivial, especially when the dataset consists of partial orders. In such cases, the parameter of the model may not be even identifiable. In this paper, we focus on three popular structures of partial orders: ranked top-l_1, l_2-way, and choice data over a subset of alternatives. We prove that when the dataset consists of combinations of ranked top-l_1 and l_2-way (or choice data over up to l_2 alternatives), mixture of k Plackett-Luce models is not identifiable when l_1+l_2< 2k-1 (l_2 is set to 1 when there are no l_2-way orders). We also prove that under some combinations, including ranked top-3, ranked top-2 plus 2-way, and choice data over up to 4 alternatives, mixtures of two Plackett-Luce models are identifiable. Guided by our theoretical results, we propose efficient generalized method of moments (GMM) algorithms to learn mixtures of two Plackett-Luce models, which are proven consistent. Our experiments demonstrate the efficacy of our algorithms. Moreover, we show that when full rankings are available, learning from different marginal events (partial orders) provides tradeoffs between statistical efficiency and computational efficiency.
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