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1. Accelerated Spectral Ranking

   Agarwal, Arpit ; Patil, Prathamesh ; Agarwal, Shivani ( January 2018 , Proceedings of the 35th International Conference on Machine Learning)

   The problem of rank aggregation from pairwise and multiway comparisons has a wide range of implications, ranging from recommendation systems to sports rankings to social choice. Some of the most popular algorithms for this problem come from the class of spectral ranking algorithms; these include the rank centrality (RC) algorithm for pairwise comparisons, which returns consistent estimates under the Bradley-Terry-Luce (BTL) model for pairwise comparisons (Negahban et al., 2017), and its generalization, the Luce spectral ranking (LSR) algorithm, which returns consistent estimates under the more general multinomial logit (MNL) model for multiway comparisons (Maystre & Grossglauser, 2015). In this paper, we design a provably faster spectral ranking algorithm, which we call accelerated spectral ranking (ASR), that is also consistent under the MNL/BTL models. Our accelerated algorithm is achieved by designing a random walk that has a faster mixing time than the random walks associated with previous algorithms. In addition to a faster algorithm, our results yield improved sample complexity bounds for recovery of the MNL/BTL parameters: to the best of our knowledge, we give the first general sample complexity bounds for recovering the parameters of the MNL model from multiway comparisons under any (connected) comparison graph (and improve significantly over previous bounds for the BTL model for pairwise comparisons). We also give a message-passing interpretation of our algorithm, which suggests a decentralized distributed implementation. Our experiments on several real-world and synthetic datasets confirm that our new ASR algorithm is indeed orders of magnitude faster than existing algorithms. 

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2. Rank Aggregation from Pairwise Comparisons in the Presence of Adversarial Corruptions

   Agarwal, Arpit ; Agarwal, Shivani ; Khanna, Sanjeev ; Patil, Prathamesh ( January 2020 , Proceedings of the 37th International Conference on Machine Learning)

   Rank aggregation from pairwise preferences has widespread applications in recommendation systems and information retrieval. Given the enormous economic and societal impact of these applications, and the consequent incentives for malicious players to manipulate ranking outcomes in their favor, an important challenge is to make rank aggregation algorithms robust to adversarial manipulations in data. In this paper, we initiate the study of robustness in rank aggregation under the popular Bradley-Terry-Luce (BTL) model for pairwise comparisons. We consider a setting where pairwise comparisons are initially generated according to a BTL model, but a fraction of these comparisons are corrupted by an adversary prior to being reported to us. We consider a strong contamination model, where an adversary having complete knowledge of the initial truthful data and the underlying true BTL parameters, can subsequently corrupt the truthful data by inserting, deleting, or changing data points. The goal is to estimate the true score/weight of each item under the BTL model, even in the presence of these corruptions. We characterize the extent of adversarial corruption under which the true BTL parameters are uniquely identifiable. We also provide a novel pruning algorithm that provably cleans the data of adversarial corruption under reasonable conditions on data generation and corruption. We corroborate our theory with experiments on both synthetic as well as real data showing that previous algorithms are vulnerable to even small amounts of corruption, whereas our algorithm can clean a reasonably high amount of corruption. 

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3. Principled Reinforcement Learning with Human Feedback from Pairwise or K-wise Comparisons

   Zhu, Banghua and ( January 2023 , Proceedings of Machine Learning Research)

   Krause, Andreas and (Ed.)

   We provide a theoretical framework for Reinforcement Learning with Human Feedback (RLHF). We show that when the underlying true reward is linear, under both Bradley-Terry-Luce (BTL) model (pairwise comparison) and Plackett-Luce (PL) model ($K$-wise comparison), MLE converges under certain semi-norm for the family of linear reward. On the other hand, when training a policy based on the learned reward model, we show that MLE fails while a pessimistic MLE provides policies with good performance under certain coverage assumption. We also show that under the PL model, both the true MLE and a different MLE which splits the $K$-wise comparison into pairwise comparisons converge, while the true MLE is asymptotically more efficient. Our results validate the empirical success of the existing RLHF algorithms, and provide new insights for algorithm design. Our analysis can also be applied for the problem of online RLHF and inverse reinforcement learning. 

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4. Lagrangian Inference for Ranking Problems

   https://doi.org/doi.org/10.1287/opre.2022.2313  

   Liu, Yue ; Fang Ethan ; Lu, Junwei ( June 2022 , Operations research)

   We propose a novel combinatorial inference framework to conduct general uncertainty quantification in ranking problems. We consider the widely adopted Bradley-Terry-Luce (BTL) model, where each item is assigned a positive preference score that determines the Bernoulli distributions of pairwise comparisons’ outcomes. Our proposed method aims to infer general ranking properties of the BTL model. The general ranking properties include the “local” properties such as if an item is preferred over another and the “global” properties such as if an item is among the top K-ranked items. We further generalize our inferential framework to multiple testing problems where we control the false discovery rate (FDR) and apply the method to infer the top-K ranked items. We also derive the information-theoretic lower bound to justify the minimax optimality of the proposed method. We conduct extensive numerical studies using both synthetic and real data sets to back up our theory. 

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5. Minimax Bounds for Generalized Pairwise Comparisons

   Lee, Kuan-Yun ; Courtade, Thomas A ( July 2021 , 2021 International Conference on Machine Learning (ICML) Workshop on Information-Theoretic Methods for Rigorous, Responsible, and Reliable Machine Learning)

   null (Ed.)

   We introduce the General Pairwise Model (GPM), a general parametric framework for pairwise comparison. Under the umbrella of the exponential family, the GPM unifies many pop- ular models with discrete observations, including the Thurstone (Case V), Berry-Terry-Luce (BTL) and Ordinal Models, along with models with continuous observations, such as the Gaussian Pairwise Cardinal Model. Using information theoretic techniques, we establish minimax lower bounds with tight topological dependence. When applied as a special case to the Ordinal Model, our results uniformly improve upon previously known lower bounds and confirms one direction of a conjecture put forth by Shah et al. (2016). Performance guarantees of the MLE for a broad class of GPMs with subgaussian assumptions are given and compared against our lower bounds, showing that in many natural settings the MLE is optimal up to constants. Matching lower and upper bounds (up to constants) are achieved by the Gaussian Pairwise Cardinal Model, suggesting that our lower bounds are best-possible under the few assumptions we adopt. 

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