DP-PCA: Statistically Optimal and Differentially Private PCA
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We study the canonical statistical task of computing the principal component from n i.i.d. data in d dimensions under (ε,δ)-differential privacy. Although extensively studied in literature, existing solutions fall short on two key aspects: (i) even for Gaussian data, existing private algorithms require the number of samples n to scale super-linearly with d, i.e., n=Ω(d^3/2), to obtain non-trivial results while non-private PCA requires only n=O(d), and (ii) existing techniques suffer from a non-vanishing error even when the randomness in each data point is arbitrarily small. We propose DP-PCA, which is a single-pass algorithm that overcomes both limitations. It is based on a private minibatch gradient ascent method that relies on private mean estimation, which adds minimal noise required to ensure privacy by adapting to the variance of a given minibatch of gradients. For sub-Gaussian data, we provide nearly optimal statistical error rates even for n=Õ(d). Furthermore, we provide a lower bound showing that sub-Gaussian style assumption is necessary in obtaining the optimal error rate.
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