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We present two language models based upon an “immediate-head” parser — our name for a parser that conditions all events below a constituent c upon the head of c. While all of the most accurate statistical parsers are of the immediate-head variety, no previous grammatical language model uses this technology. The perplexity for both of these models significantly improve upon the trigram model base-line as well as the best previous grammarbased language model. | Immediate-Head Parsing for Language Models Eugene Charniak Brown Laboratory for Linguistic Information Processing Department of Computer Science Brown University Box 1910 Providence RI ec@cs.brown.edu Abstract We present two language models based upon an immediate-head parser our name for a parser that conditions all events below a constituent c upon the head of c. While all of the most accurate statistical parsers are of the immediate-head variety no previous grammatical language model uses this technology. The perplexity for both of these models significantly improve upon the trigram model base-line as well as the best previous grammarbased language model. For the better of our two models these improvements are 24 and 14 respectively. We also suggest that improvement of the underlying parser should significantly improve the model s perplexity and that even in the near term there is a lot of potential for improvement in immediate-head language models. 1 Introduction All of the most accurate statistical parsers 1 3 6 7 12 14 are lexicalized in that they condition probabilities on the lexical content of the sentences being parsed. Furthermore all of these This research was supported in part by NSF grant LIS SBR 9720368 and by NSF grant 00100203 IISO085980. The author would like to thank the members of the Brown Laboratory for Linguistic Information Processing BLLIP and particularly Brian Roark who gave very useful tips on conducting this research. Thanks also to Fred Jelinek and Ciprian Chelba for the use of their data and for detailed comments on earlier drafts of this paper. parsers are what we will call immediate-head parsers in that all of the properties of the immediate descendants of a constituent c are assigned probabilities that are conditioned on the lexical head of c. For example in Figure 1 the probability that the vp expands into v np pp is conditioned on the head of the vp put as are the choices of the sub-heads under the vp i.e. ball the head of the np
