Recovery of Empty Nodes in Parse Structures

In this paper, we describe a new algorithm for recovering WH-trace empty nodes. Our approach combines a set of hand-written patterns together with a probabilistic model. Because the patterns heavily utilize regular expressions, the pertinent tree structures are covered using a limited number of patterns. The probabilistic model is essentially a probabilistic context-free grammar (PCFG) approach with the patterns acting as the terminals in production rules. We evaluate the algorithm’s performance on gold trees and parser output using three different metrics. Our method compares favorably with state-of-the-art algorithms that recover WH-traces.