Use syntax phrases of different level to improve BoW

Phrases of different level in parse tree have different level of semantic abstract, and may function diversely in classification. This paper uses syntax phrases of different level to improve BoW representation. The result shows that level of phrases is valuable to capture the commonness of positive instances, and can better the discernment of positive instances. But it decreases the discernment of negative instances on the other side. And we also find BoW is sufficient as for negative instance recognition if there are enough positive instances.