Dependency-based empty category detection via phrase structure trees

Nianwen Xue and Yaqin Yang

We describe a novel approach to detecting empty categories (EC) as represented in dependency trees as well as a new metric for measuring EC detection accuracy. The new metric takes into account not only the position and type of an EC, but also the head it is a dependent of in a dependency tree. We also introduce a variety of new features that are more suited for this approach. Tested on a subset of the Chinese Treebank, our system improved significantly over the best previously reported results even when evaluated with this new, arguably more stringent metric.