tailieunhanh - Báo cáo khoa học: "Learning Accurate, Compact, and Interpretable Tree Annotation"

We present an automatic approach to tree annotation in which basic nonterminal symbols are alternately split and merged to maximize the likelihood of a training treebank. Starting with a simple Xbar grammar, we learn a new grammar whose nonterminals are subsymbols of the original nonterminals. | Learning Accurate Compact and Interpretable Tree Annotation Slav Petrov Leon Barrett Romain Thibaux Dan Klein Computer Science Division EECS Department University of California at Berkeley Berkeley CA 94720 petrov lbarrett thibaux klein @ Abstract We present an automatic approach to tree annotation in which basic nonterminal symbols are alternately split and merged to maximize the likelihood of a training treebank. Starting with a simple X-bar grammar we learn a new grammar whose nonterminals are subsymbols of the original nonterminals. In contrast with previous work we are able to split various terminals to different degrees as appropriate to the actual complexity in the data. Our grammars automatically learn the kinds of linguistic distinctions exhibited in previous work on manual tree annotation. On the other hand our grammars are much more compact and substantially more accurate than previous work on automatic annotation. Despite its simplicity our best grammar achieves an Fl of on the Penn Treebank higher than fully lexicalized systems. 1 Introduction Probabilistic context-free grammars PCFGs underlie most high-performance parsers in one way or another Collins 1999 Charniak 2000 Charniak and Johnson 2005 . However as demonstrated in Charniak 1996 and Klein and Manning 2003 a PCFG which simply takes the empirical rules and probabilities off of a treebank does not perform well. This naive grammar is a poor one because its context-freedom assumptions are too strong in some places . it assumes that subject and object NPs share the same distribution and too weak in others . it assumes that long rewrites are not decomposable into smaller steps . Therefore a variety of techniques have been developed to both enrich and generalize the naive grammar ranging from simple tree annotation and symbol splitting Johnson 1998 Klein and Manning 2003 to full lexicalization and intricate smoothing Collins 1999 Charniak 2000 . In this paper we .