Differentially Private Matrix Completion, Revisited
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We study the problem of privacy-preserving collaborative filtering where the objective is to reconstruct the entire users-items preference matrix using a few observed preferences of users for some of the items. Furthermore, the collaborative filtering algorithm should reconstruct the preference matrix while preserving the privacy of each user. We study this problem in the setting of joint differential privacy where each user computes her own preferences for all the items, without violating privacy of other users' preferences. We provide the first provably differentially private algorithm with formal utility guarantees for this problem. Our algorithm is based on the Frank-Wolfe (FW) method, and consistently estimates the underlying preference matrix as long as the number of users m is ω(n^5/4), where n is the number of items, and each user provides her preference for at least √(n) randomly selected items. We also empirically evaluate our FW-based algorithm on a suite of datasets, and show that our method provides nearly same accuracy as the state-of-the-art non-private algorithm, and outperforms the state-of-the-art private algorithm by as much as 30
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