  Recent semi-supervised learning (SSL) methods typically include a filtering
strategy to improve the quality of pseudo labels. However, these filtering
strategies are usually hand-crafted and do not change as the model is updated,
resulting in a lot of correct pseudo labels being discarded and incorrect
pseudo labels being selected during the training process. In this work, we
observe that the distribution gap between the confidence values of correct and
incorrect pseudo labels emerges at the very beginning of the training, which
can be utilized to filter pseudo labels. Based on this observation, we propose
a Self-Adaptive Pseudo-Label Filter (SPF), which automatically filters noise in
pseudo labels in accordance with model evolvement by modeling the confidence
distribution throughout the training process. Specifically, with an online
mixture model, we weight each pseudo-labeled sample by the posterior of it
being correct, which takes into consideration the confidence distribution at
that time. Unlike previous handcrafted filters, our SPF evolves together with
the deep neural network without manual tuning. Extensive experiments
demonstrate that incorporating SPF into the existing SSL methods can help
improve the performance of SSL, especially when the labeled data is extremely
scarce.
