Constituents Correspond to Word Sequence Patterns among Sentences with Equivalent Predicate-Argument Structures: Unsupervised Constituency Parsing by Span Matching

Unsupervised constituency parsing is about identifying word sequences that form a syntactic unit (i.e., constituents) in a target sentence. Linguists identify the constituent by evaluating a set of Predicate-Argument Structure (PAS) equivalent sentences where we find the constituent corresponds to frequent word sequences. However, such information is unavailable to previous parsing methods which identify the constituent by observing sentences with diverse PAS. In this study, we empirically verify that constituents correspond to word sequence patterns in the PAS-equivalent sentence set. We propose a frequency-based method span-overlap, applying the word sequence pattern to computational unsupervised parsing for the first time. Parsing experiments show that the span-overlap parser outperforms state-of-the-art parsers in eight out of ten languages. Further discrimination analysis confirms that the span-overlap method can non-trivially separate constituents from non-constituents. This result highlights the utility of the word sequence pattern. Additionally, we discover a multilingual phenomenon: participant-denoting constituents are more frequent than event-denoting constituents. The phenomenon indicates a behavioral difference between the two constituent types, laying the foundation for future labeled unsupervised parsing.