  Multi-task learning (MTL) has become increasingly popular in natural language
processing (NLP) because it improves the performance of related tasks by
exploiting their commonalities and differences. Nevertheless, it is still not
understood very well how multi-task learning can be implemented based on the
relatedness of training tasks. In this survey, we review recent advances of
multi-task learning methods in NLP, with the aim of summarizing them into two
general multi-task training methods based on their task relatedness: (i) joint
training and (ii) multi-step training. We present examples in various NLP
downstream applications, summarize the task relationships and discuss future
directions of this promising topic.
