这篇论文把合成数据增强对不平衡分类的效果掰开了讲：模型对的时候加数据没提升，模型错的时候加数据有救但别指望线性增益。
该论文建立了一个理论框架，分析合成数据增强对基于分数的分类指标（AUROC、AUPRC、最佳阈值平衡准确率、最佳阈值F1分数）的影响。将增强效应分解为类权重变化和合成数据与真实分布之间的偏差。在模型设定正确时，增强无法提供总体改进，仅可能减少有限样本方差，但会因合成分布误差引入额外偏差。在模型错误指定时，增强可通过改变有效类平衡来纠正原始不平衡目标下的排序错误。仿真结果表明，设定正确时收益有限，错误指定时存在非单调改善。
When Does Synthetic Data Augmentation Improve Score-Based Imbalanced Classification?
Synthetic data augmentation is widely used to mitigate class imbalance, but its theoretical effects on score-based classification remain poorly understood. This paper develops a framework for characterizing when synthetic minority augmentation can improve threshold-integrated and threshold-optimized metrics, including AUROC, AUPRC, best-threshold balanced accuracy, and best-threshold \(\F_1\) score. We separate the effect of augmentation into two components: a change in effective class weighting and a discrepancy between the synthetic and true minority distributions. Under well-specified score models, the raw estimator already targets the likelihood-ratio ordering, which is population-optimal for the metrics considered. Consequently, augmentation cannot provide a fundamental population-level improvement beyond possible finite-sample variance reduction, and may introduce additional bias through synthetic distributional error. We further establish minimax lower bounds showing that the raw estimator already achieves the optimal metric-regret rate in the well-specified regime. Under misspecification, however, augmentation can play a qualitatively different role: by changing the effective class balance, it can alter the restricted-class projection and correct ranking errors induced by the raw imbalanced objective. We provide explicit improvement bounds quantifying the roles of approximation error, finite-sample estimation error, and synthetic distributional error. Simulation studies corroborate the theory, demonstrating limited gains under well-specification and nontrivial but nonmonotone improvements under misspecification.