Dataset Identity, Not Novelty: The Source of an Inflated OOD Detection Gain
This study addresses the inflated out-of-distribution (OOD) detection performance in existing methods, which stems from models fitting dataset identity rather than genuine novelty. To resolve this, we introduce a novel whole-dataset hold-out protocol that decouples identity bias from novelty bias. By integrating posterior detection, feature-space directional fitting, and closed-form mathematical derivations, we quantify the inflation of reported predictive gains. Our analysis reveals that conventionally reported improvements are largely spurious and that model capacity is not the primary contributing factor. Furthermore, we establish that a single constant baseline serves as an upper bound for genuine performance gains. This baseline remains robust on a predefined validation set, effectively recalibrating the evaluation standards within the field.