Dynamic Re-weighting for Long-tailed Semi-supervised Learning

Semi-supervised Learning (SSL) reduces significant human annotations by simply demanding a small number of labelled samples and a large number of unlabelled samples. The research community has often developed SSL regarding the nature of a balanced data set; in contrast, real data is often imbalanced or even long-tailed. The need to study SSL under imbalance is therefore critical. In this paper, we essentially extend FixMatch (a SSL method) to the imbalanced case. We find that the unlabeled data is as well highly imbalanced during the training process; in this respect we propose a re-weighting solution based on the effective number. Furthermore, since prediction uncertainty leads to temporal variations in the number of pseudo-labels, we are innovative in proposing a dynamic reweighting scheme on the unlabeled data. The simplicity and validity of our method are backed up by experimental evidence. Especially on CIFAR-10, CIFAR-100, ImageNet127 data sets, our approach provides the strongest results against previous methods across various scales of imbalance.