A Systematic Analysis of Out-of-Distribution Detection Under Representation and Training Paradigm Shifts

TL;DR

This paper systematically benchmarks out-of-distribution detection methods across various models, datasets, and training paradigms, revealing the importance of learned representations and proposing new analysis tools and improvements.

Contribution

It introduces a comprehensive benchmark framework, analyzes representation effects on detection, and proposes PCA filtering and NC-based prediction methods for OOD detection.

Findings

Simple probabilistic scores outperform complex ones across models.

Margin-based scores excel in near-OOD regimes, while geometry-aware scores perform better with severe shifts.

Representation collapse correlates with the effectiveness of boundary-aware scores.

Abstract

We present a systematic benchmark of out-of-distribution (OOD) detection CSFs through a representation-centric lens. Our study spans CNN and ViT backbones, multiple training paradigms, four image-classification source datasets (CIFAR-10, CIFAR-100, SuperCIFAR-100, and TinyImageNet), and OOD datasets grouped into near, mid, and far regimes using CLIP-derived semantic distances. To compare CSFs across these settings, we employ a multiple-comparison-controlled rank pipeline that identifies top cliques of statistically indistinguishable winners under threshold-free ranking metrics (AURC and AUGRC). The main empirical finding is that the competitive detector family depends more on the learned representation than on score design alone. For both CNNs and ViTs, simple probabilistic scores dominate misclassification detection. On CNNs, margin-based scores are strongest in near-OOD regimes, while geometry-aware scores such as NNGuide, fDBD, and CTM become more competitive as shift severity increases. On fine-tuned ViTs, the top cliques are led mainly by reconstruction- and residual-based scores. To interpret these ranking shifts, we analyze the last-layer representation using Neural Collapse (NC) metrics. The resulting picture is consistent across architectures: prototype- and boundary-aware scores become stronger when the representation is more collapsed and better aligned with classifier weights, whereas weaker-collapse regimes favor gradient- and manifold-based scores. Building on these insights, we propose two contributions: a simple PCA-based projection-filtering procedure that improves detector performance, and an approach that uses NC measurements computed from a trained classifier to predict its competitive out-of-distribution detector shortlist, without requiring any additional OOD data.