Counting Distinct Elements Under Person-Level Differential Privacy

Thomas Steinke, Alexander Knop

Keywords: differential privacy, user-level privacy, person-level privacy, sensitivity

TL;DR: We show how to privately compute the number of unique elements, despite this having infinite sensitivity in the user-level DP setting.

Abstract:

We study the problem of counting the number of distinct elements in a dataset subject to the constraint of differential privacy.
We consider the challenging setting of person-level DP (a.k.a. user-level DP) where each person may contribute an unbounded number of items and hence the sensitivity is unbounded.

Our approach is to compute a bounded-sensitivity version of this query, which reduces to solving a max-flow problem.
The sensitivity bound is optimized to balance the noise we must add to privatize the answer against the error of the approximation of the bounded-sensitivity query to the true number of unique elements.

Submission Number: 24