Count-mean Sketch as an Optimized Framework for Frequency Estimation with Local Differential Privacy

Abstract: This paper identifies that a group of state-of-the-art locally-differentially-private (LDP) algorithms for frequency estimation are equivalent to the private Count-Mean Sketch (CMS) algorithm with different parameters. Therefore, we revisit the private CMS, correct errors in the original CMS paper regarding expectation and variance, modify the CMS implementation to eliminate existing bias, and explore optimized parameters for CMS to achieve optimality in reducing the worst-case mean squared error (MSE), $l_1$ loss, and $l_2$ loss. Additionally, we prove that pairwise-independent hashing is sufficient for CMS, reducing its communication cost to the logarithm of the cardinality of all possible values (i.e., a dictionary). As a result, the aforementioned optimized CMS is proven theoretically and empirically to be the only algorithm optimized for reducing the worst-case MSE, $l_1$ loss, and $l_2$ loss when dealing with a very large dictionary. Furthermore, we demonstrate that randomness is necessary to ensure the correctness of CMS, and the communication cost of CMS, though low, is unavoidable despite the randomness being public or private.