A Hybrid Character Representation for Chinese Event Detection

For the Chinese language, event triggers in a sentence may appear inside or across words after word segmentation. Thus recent works on Chinese event detection often formulate the task as a character-wise sequence labeling problem instead of a word-wise one. Due to a limited amount of corpus, however, it is more difficult in practice to train character-wise models to capture the inner structure of event triggers and the semantics of sentence-level context compared with word-wise ones. In this paper, we propose to improve character-wise models by incorporating word information and language model representation into Chinese character representation. More specifically, the former consists of the position of the character inside a word and the word's embedding, which can aid structural pattern learning; the latter is obtained by BERT, which contains long-distance semantic information. We construct a sequence tagging model equipped with the hybrid representation and evaluate our model on ACE 2005 Chinese corpus. Experiment results show that both word information and language model representation are effective enhancements, and our model gains an increase of 4.5 (6.5%) and 6.1 (9.4%) in F1-score in event trigger identification task and classification task respectively over the state-of-the-art method.