Unifying Distribution Alignment as a Loss for Imbalanced Semi-supervised Learning

Justin Lazarow, Kihyuk Sohn, Chun-Liang Li, Zizhao Zhang, Chen-Yu Lee, Tomas Pfister

Keywords: semi-supervised learning, imbalanced learning

Abstract: While remarkable progress in imbalanced supervised learning has been made recently, less attention has been given to the setting of imbalanced semi-supervised learning (SSL) where not only is a few labeled data provided, but the underlying data distribution can be severely imbalanced. Recent works require both complicated sampling-based strategies of pseudo-labeled data and distribution alignment of the pseudo-label distribution to accommodate this imbalance. We present a novel approach that relies only on a form of a distribution alignment but no sampling strategy where rather than aligning the pseudo-labels during inference, we move the distribution alignment component into the respective cross entropy loss computations for both the supervised and unsupervised losses. This alignment compensates for both imbalance in the data as well as the eventual distributional shift present during evaluation. Altogether, this provides a single, unified strategy that offers both significantly reduced training requirements and improved performance across both low and richly labeled regimes and over varying degrees of imbalance. In experiments, we validate the efficacy of our method on SSL variants of CIFAR10-LT, CIFAR100-LT, and ImageNet-127. On ImageNet-127, our method shows 1.6% accuracy improvement over the previous best method with 80% training time reduction.

One-sentence Summary: We simplify imbalanced semi-supervised learning by using the same alignment approach in both supervised and unsupervised branches --- requiring only a few lines of code, reducing training time, and improving classification performance overall