Differentially Private Gradient Boosting on Linear Learners for Tabular Data

Gradient boosting takes \emph{linear} combinations of weak base learners. Therefore, absent privacy constraints (when we can exactly optimize over the base models) it is not effective when run over base learner classes that are closed under linear combinations (e.g. linear models). As a result, gradient boosting is typically implemented with tree base learners (e.g., XGBoost), and this has become the state of the art approach in tabular data analysis. Prior work on private gradient boosting focused on taking the state of the art algorithm in the non-private regime---boosting on trees---and making it differentially private. Surprisingly, we find that when we use differentially private learners, gradient boosting over trees is not as effective as gradient boosting over linear learners. In this paper, we propose differentially private gradient-boosted linear models as a private classification method for tabular data. We empirically demonstrate that, under strict privacy constraints, it yields higher F1 scores than the private versions of gradient-boosted trees on five real-world binary classification problems. This work adds to the growing picture that the most effective learning methods under differential privacy may be quite different from the most effective learning methods without privacy.