Epistemic Parity: Reproducibility as an Evaluation Metric for Differential Privacy

Differential privacy (DP) data synthesizers are increasingly proposed to afford public release of sensitive information, offering theoretical guarantees for privacy (and, in some cases, utility), but limited empirical evidence of utility in practical settings. Utility is typically measured as the error on representative proxy tasks, such as descriptive statistics, multivariate correlations, the accuracy of trained classifiers, or performance over a query workload. The ability of these results to generalize to practitioners’ experience has been questioned in a number of settings, including the U.S. Census. In this paper, we propose an evaluation methodology for synthetic data that avoids assumptions about the representativeness of proxy tasks, instead measuring the likelihood that published conclusions would change had the authors used synthetic data, a condition we call epistemic parity. Our methodology consists of reproducing empirical conclusions of peer-reviewed papers on real, publicly available data, then re-running these experiments a second time on DP synthetic data and comparing the results. We instantiate our methodology over a benchmark of recent peer-reviewed papers in the social sciences. We express the authors’ claims computationally to automate the experiment, generate DP synthetic datasets using multiple state-of-the-art mechanisms, then estimate the likelihood that these conclusions will hold. We find that, for reasonable privacy regimes, DP synthesizers can achieve high epistemic parity for several papers in our benchmark. However, some papers, and particularly some specific findings, are difficult to reproduce for any of the synthesizers. Given these results, we recommend a new class of mechanisms that offer stronger utility guarantees (as measured by epistemic parity) and more nuanced privacy protection using application-specific risks and threat models.