  Many natural language processing (NLP) tasks are naturally imbalanced, as
some target categories occur much more frequently than others in the real
world. In such scenarios, current NLP models still tend to perform poorly on
less frequent classes. Addressing class imbalance in NLP is an active research
topic, yet, finding a good approach for a particular task and imbalance
scenario is difficult.
  With this survey, the first overview on class imbalance in deep-learning
based NLP, we provide guidance for NLP researchers and practitioners dealing
with imbalanced data. We first discuss various types of controlled and
real-world class imbalance. Our survey then covers approaches that have been
explicitly proposed for class-imbalanced NLP tasks or, originating in the
computer vision community, have been evaluated on them. We organize the methods
by whether they are based on sampling, data augmentation, choice of loss
function, staged learning, or model design. Finally, we discuss open problems
such as dealing with multi-label scenarios, and propose systematic benchmarking
and reporting in order to move forward on this problem as a community.
