Optimal Private Median Estimation under Minimal Distributional Assumptions
Christos Tzamos, Manolis Vlatakis-Gkaragkounis, Ilias Zadik
Spotlight presentation: Orals & Spotlights Track 10: Social/Privacy
on 2020-12-08T07:50:00-08:00 - 2020-12-08T08:00:00-08:00
on 2020-12-08T07:50:00-08:00 - 2020-12-08T08:00:00-08:00
Toggle Abstract Paper (in Proceedings / .pdf)
Abstract: We study the fundamental task of estimating the median of an underlying distribution from a finite number of samples, under pure differential privacy constraints. We focus on distributions satisfying the minimal assumption that they have a positive density at a small neighborhood around the median. In particular, the distribution is allowed to output unbounded values and is not required to have finite moments. We compute the exact, up-to-constant terms, statistical rate of estimation for the median by providing nearly-tight upper and lower bounds. Furthermore, we design a polynomial-time differentially private algorithm which provably achieves the optimal performance. At a technical level, our results leverage a Lipschitz Extension Lemma which allows us to design and analyze differentially private algorithms solely on appropriately defined ``typical" instances of the samples.