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Beyond PCFGs

Beyond PCFGs. Chris Brew. Ohio State University. Beyond PCFGs. Shift-reduce parsers probabilistic LR parsers Data-oriented parsers. Motivation. Get round the limitations of the PCFG model Exploit knowledge about individual words Build better language models. Shift-reduce.

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Beyond PCFGs

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  1. Beyond PCFGs Chris Brew Ohio State University

  2. Beyond PCFGs • Shift-reduce parsers • probabilistic LR parsers • Data-oriented parsers

  3. Motivation • Get round the limitations of the PCFG model • Exploit knowledge about individual words • Build better language models

  4. Shift-reduce • Simple version either shift a word from the input list to the parse stack or reduce two elements from top of parse stack to a single tree • Hermjakob and Mooney cmp-lg 9706002 • structures rather than just trees and words • more complex parse action language • not just binary rules

  5. Machine learning for shift-reduce • supervisor shows the system correct sequences of parsing actions • system tries to learn to predict correct actions • needs a feature language • as it learns, the supervisor has less need to override the actions chosen by the system.

  6. Examples of the feature language • broad syntactic class of the third element on the stack • the tense of the first element of the input list • Does top element of stack contain an object? • Could top frame be an adjectival degree adverb (e.g. very)? • Is frame1 a possible agent/patient of frame2? • Do frame1 and frame2 satisfy subject-verb agreement?

  7. Hand-crafted knowledge used • 205 features, all moderately local (no references to 1000th element of the stack or anything like that) • 4356 node lexical knowledge base • subcategorisation table for 242 verbs • But we learn the association between features and actions

  8. Various different hybrid decision structures • best was a hierarchical list of decision trees which encoded information about the task. Schematically. • decide whether to do anything • if not, we are done • if so, decide whether to do a reduction • if so, decide which reduction • if not, decide what sort of shift to do

  9. Evaluation • Corpus of 272 annotated sentences. • 17-fold cross validation (17 blocks of 16 sentences each) • Precision of 92.7%, recall of 92.8%: average length 17.1 words, with 43.5 parse actions per sentence. Parseval measures. • Correct structure and labelling 26.8% (i.e. 1 in 4 sentences are completely correct)

  10. Comments on Hermjakob and Mooney • A lot of grunt work needed - but not as much as full rationalist NLP system • The knowledge used is micro-modular very small pieces of highly independent knowledge • Test set is small, sentences short • Fairly robust • Good on small scale tests in an English/German MT task

  11. N N N N N N N N Probabilistic LR Parsers • Briscoe and Carroll CL 19(1) pp 25-59) • PCFGsgive these subtrees same probability N N

  12. LR Parsing • Builds a parsing table which gives parsing actions and Gotos for possible combinations of parser state and input symbols • There may be parsing action conflicts, in which more than one action is available. • In programming language grammars, you almost never want conflicts. • In NL grammars, you have no escape!

  13. Probabilistic LR • When there is a conflict, non-deterministically execute all possible actions. • But score them according to a probability distribution. • So where do the probabilities come from? And what do they mean? See analysis in Stolcke’s paper relating them to his forward and inner probabilities.

  14. LR parsing using Alvey Tools Grammar • Wide coverage unification grammar written by Claire Grover and Ted Briscoe • Build LR tables from CF backbone of this grammar • Interactively build disambiguated training corpus by supervising choice of parse actions

  15. Evaluation • Very good performance on LDOCE noun definitions 76% correct structure and labelling • State of the art results in later work on tag sequence grammars where the available lexical information is more restricted. (54% correct structure and labelling) • Work underway to bring this technique to Wall Street Journal data for comparison with other methods

  16. Data-oriented parsing • Rens Bod: Enriching Linguistics with Statistics: Performance Models of Natural Language, Amsterdam Ph.D • Treebank data again (this time ATIS -- 600 sentences) • Radical rejection of context-free assumption • Count subtrees of arbitrary depth, not rule applications

  17. A corpus

  18. NP V NP Euan likes Matthew S S S NP VP S NP VP NP VP Matthew V NP NP VP V NP Euan Matthew Tree fragments Some of the fragments S NP VP Matthew V NP likes Euan

  19. The probability of a tree • The probability of all the ways of making it out of fragments • The probability of a fragment is given as a ratio between the frequency of the fragment and the total frequency of all fragments having that root

  20. Complexity • It’s hard to find efficient algorithms for sewing together DOP trees (cf. Si’maan for solutions) • Only very small corpora feasible • In practice, depth may have to be limited. • Many tree fragments are very rare, so there is an issue about smoothing

  21. Evaluation • several variations studied, DOP4 geta parse accuracies around 80% without a hand-coded dictionary, DOP-5 around 90% with. • results to be interpreted with caution due to small size of corpus • Evaluation on Dutch OVIS domain suggests that DOP is not competitive with Groningen’s more labour intensive system (but maybe that’s not the point)

  22. Where to find out more • Papers by Bod, Carroll, Hermjakob. • Manning and Schütze ch 12. • http://xxx.soton.ac.uk/archive/cs/intro.html (subarea Computation and Language)

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