The Impact of the Single-Label Assumption in Image Recognition Benchmarking

Deep neural networks (DNNs) are typically evaluated under the assumption that each image has a single correct label. However, many images in benchmarks like ImageNet contain multiple valid labels, creating a mismatch between evaluation protocols and the actual complexity of visual data. This mismatch can penalize DNNs for predicting correct but unannotated labels, which may partly explain reported accuracy drops, such as the widely cited 11 to 14 percent top-1 accuracy decline on ImageNetV2, a replication test set for ImageNet. This raises the question: do such drops reflect genuine generalization failures or artifacts of restrictive evaluation metrics? We rigorously assess the impact of multi-label characteristics on reported accuracy gaps. To evaluate the multi-label prediction capability (MLPC) of single-label-trained models, we introduce a variable top-$k$ evaluation, where $k$ matches the number of valid labels per image. Applied to 315 ImageNet-trained models, our analyses demonstrate that conventional top-1 accuracy disproportionately penalizes valid but secondary predictions. We also propose Aggregate Subgroup Model Accuracy (ASMA) to better capture multi-label performance across model subgroups. Our results reveal wide variability in MLPC, with some models consistently ranking multiple correct labels higher. Under this evaluation, the perceived gap between ImageNet and ImageNetV2 narrows substantially. To further isolate multi-label recognition performance from contextual cues, we introduce PatchML, a synthetic dataset containing systematically combined object patches. PatchML demonstrates that many models trained with single-label supervision nonetheless recognize multiple objects. Altogether, these findings highlight limitations in single-label evaluation and reveal that modern DNNs have stronger multi-label capabilities than standard metrics suggest.

Esla Timothy Anzaku, Seyed Amir Mousavi, Arnout Van Messem, Wesley De Neve · 2024