An Improved Nested Named-Entity Recognition Model for Subject Recognition Task under Knowledge Base Question Answering

In the subject recognition (SR) task under Knowledge Base Question Answering (KBQA), a common method is by training and employing a general flat Named-Entity Recognition (NER) model. However, it is not effective and robust enough in the case that the recognized entity could not be strictly matched to any subjects in the Knowledge Base (KB). Compared to flat NER models, nested NER models show more flexibility and robustness in general NER tasks, whereas it is difficult to employ a nested NER model directly in an SR task. In this paper, we take advantage of features of a nested NER model and propose an Improved Nested NER Model (INNM) for the SR task under KBQA. In our model, each question token is labeled as either an entity token, a start token, or an end token by a modified nested NER model based on semantics. Then, entity candidates would be generated based on such labels, and an approximate matching strategy is employed to score all subjects in the KB based on string similarity to find the best-matched subject. Experimental results show that our model is effective and robust to both single-relation questions and complex questions, which outperforms the baseline flat NER model by a margin of 3.3% accuracy on the SimpleQuestions dataset and a margin of 11.0% accuracy on the WebQuestionsSP dataset.