Retrieving Correct Semantic Boundaries in Dependency Structure

This paper describes the retrieval of correct semantic boundaries for predicateargument structures annotated by dependency structure. Unlike phrase structure, in which arguments are annotated at the phrase level, dependency structure does not have phrases so the argument labels are associated with head words instead: the subtree of each head word is assumed to include the same set of words as the annotated phrase does in phrase structure. However, at least in English, retrieving such subtrees does not always guarantee retrieval of the correct phrase boundaries. In this paper, we present heuristics that retrieve correct phrase boundaries for semantic arguments, called semantic boundaries, from dependency trees. By applying heuristics, we achieved an F1-score of 99.54% for correct representation of semantic boundaries. Furthermore, error analysis showed that some of the errors could also be considered correct, depending on the interpretation of the annotation.