Alpha and Prejudice: Improving α-Sized Worst Case Fairness via Intrinsic Reweighting.

Abstract

Achieving worst case group fairness typically relies on maximizing the utility of the worst-off demographic group. However, in practice, demographic information is often unavailable, making direct max-min formulations infeasible. To address this, recent work introduces a relaxed setting, using a lower bound $\alpha $ on the minimal group size-referred to as " $\alpha $ -sized worst case fairness" in this article. We first motivate the importance of this setting by highlighting its relevance to data privacy, a critical yet underexplored perspective. Rather than simply retraining on worst-off samples, we propose a reweighting approach that assigns sample weights based on their intrinsic contributions to fairness. To handle the global nature of worst case objectives efficiently, we develop a stochastic learning algorithm that simplifies training without sacrificing performance. We also address the impact of outliers by introducing a robust variant of our method. Through theoretical analysis and extensive experiments on standard fairness benchmarks, we show that our methods not only connect naturally to existing fairness-through-reweighting approaches but also outperform strong baselines.