The dissertation of Andreas Stolcke is approved: University of ...

CHAPTER 4. STOCHASTIC CONTEXT-FREE GRAMMARS 103Rules learned from smaller samples tend to be useful in structuring larger samples (but not the other¡way around). Thus the analyses of the previous samples can effectively guide the search for mergesbased on the longer, more recent ones.An interesting related result from non-probabilistic language induction is that strictly (lexicographically)ordered sample presentation makes the learning problem for certain classes of grammar provably easier(Porat & Feldman 1991).4.5.4 Summary and DiscussionThe above artificial examples show that SCFG learning based purely on distributional evidence andthe generic Bayesian simplicity bias can correctly identify several of the common linguistic structures. It alsoshows that this limited evidence can be misleading, especially when it comes to finding the ‘right’ phrasestructures. This is hardly surprising, given that a lot of the cues for human judgments of linguistic structurepresumable come from other sources, such as the semantic referents of the syntactic elements, phonologicalcues, morphological markers, etc. (Morgan et al. 1987).In a brief experiment, we applied our algorithm to a 1200 sentence corpus collected with the BeRPspeech system (Jurafsky et al. 1994a). The algorithm produced mostly plausible lexical categories and a largenumber of chunks corresponding to frequent phrases and collocations. However, the generalization achievedwas nowhere near what would be required for a sufficient coverage of new data. The problems can partlybe addressed by simple preprocessing steps, such as tagging of lexical items using standard probabilisticapproaches that achieve reasonable performance on data of this sort (Kupiec 1992b). Non-traditional phrasestructuring may not be a problem if sentence-level generalization is the main goal for an application. Also,bracketing may be induced separately using bracketing models trained from structured data (Brill 1993). Weplan to investigate such hybrid strategies in the future.