Paper: Transformational Priors Over Grammars

ACL ID W02-1009
Title Transformational Priors Over Grammars
Venue Conference on Empirical Methods in Natural Language Processing
Session Main Conference
Year 2002

This paper proposes a novel class of PCFG parameterizations that support linguistically reasonable priors over PCFGs. To estimate the parameters is to discover a notion of relatedness among context-free rules such that related rules tend to have related probabilities. The prior favors grammars in which the relationships are simple to describe and have few major excep- tions. A basic version that bases relatedness on weighted edit distance yields superior smoothing of grammars learned from the Penn Treebank (20% reduction of rule perplexity over the best previous method). 1 A Sketch of the Concrete Problem This paper uses a new kind of statistical model to smooth the probabilities of PCFG rules. It focuses on “flat” or “dependency-style” rules. These resemble subcategoriza- tion fra...