When Does Data Augmentation Help with Few Chest-Radiograph Training Patients?

Scarce medical images often prompt data augmentation. We ask when it earns its opportunity cost under a complete information budget. Across four collections, classical and GAN candidates are compared with real-only training, full-pool exposure, score adjustment and direct label use. Transformations reduce within-patient variation; source mismatch and between-patient uncertainty remain. Fixed 50-model ResNet50 ensembles with ranked classical candidates and probability averaging outperform equally adjusted real-only and full-pool ensembles at five sizes. We evaluate procedures in 50 paired draws at 50, 100, 150, 200 and 250 image-training patients. The design counts 200 regularization patients plus disjoint 100-patient selection and adjustment sets. Evaluation includes 38,743 CheXpert patients. At 250 patients, guidance reduces maximum group cross-entropy by 0.0442 versus real-only training (simultaneous 95% interval, 0.0048–0.0836). Adding the 100 selection patients to each original training cohort outperforms using their labels only to guide augmentation at 50–200 patients. Thus, use independent labels first when cohorts are smallest; consider selected augmentation once patient evidence, strong representation and aggregation are available. The 250-patient result marks the upper tested setting, not a universal threshold. AUROC remains stable while sensitivity and specificity shift, requiring operating-point review.