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Probabilistic Earley Parsing

Probabilistic Earley Parsing. Charlie Kehoe, Spring 2004. Based on the 1995 paper by Andreas Stolcke: An Efficient Probabilistic Context-Free Parsing Algorithm that Computes Prefix Probabilities. Overview. What is this paper all about? Key ideas from the title: Context-Free Parsing

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Probabilistic Earley Parsing

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  1. ProbabilisticEarley Parsing Charlie Kehoe, Spring 2004 Based on the 1995 paper by Andreas Stolcke: An Efficient Probabilistic Context-Free Parsing Algorithm that Computes Prefix Probabilities

  2. Overview • What is this paper all about? • Key ideas from the title: • Context-Free Parsing • Probabilistic • Computes Prefix Probabilities • Efficient

  3. Context-Free Parsing The ball is heavy. Parser The ball is heavy

  4. Context-Free Parsing The ball is heavy. Grammar Parser The ball is heavy

  5. Context-Free Parsing The ball is heavy. Grammar Parser Lexicon The ball is heavy

  6. Context-Free Parsing • What if there are multiple legal parses? • Example: “Yair looked over the paper.” • How does the word “over” function? S S NP VP NP VP or PP N V NP N V P NP Yair looked over the paper Yair looked over the paper

  7. Probabilistic Parsing • Use probabilities to find the most likely parse • Store typical probabilities for words and rules • In this case: S S NP VP NP VP or PP N V NP N V P NP Yair looked over the paper Yair looked over the paper P = 0.99 P = 0.01

  8. Prefix Probabilities • How likely is a partial parse? S S NP VP NP VP or PP N V NP N V P NP Yair looked over … Yair looked over …

  9. Efficiency • The Earley algorithm (upon which Stolcke builds) is one of the most efficient known parsing algorithms • Probabilities allow intelligent pruning of the developing parse tree(s)

  10. Parsing Algorithms • How do we construct a parse tree? • Work from grammar to sentence (top-down) • Work from sentence to grammar (bottom-up) • Work from both ends at once (Earley) • Predictably, Earley works best

  11. Earley Parsing Overview • Start with a root constituent, e.g. sentence • Until the sentence is complete, repeatedly • Predict: expand constituents as specified in the grammar • Scan: match constituents with words in the input • Complete: propagate constituent completions up to their parents • Prediction is top-down, while scanning and completion are bottom-up

  12. Earley Parsing Overview • Earley parsing uses a chart rather than a tree to develop the parse • Constituents are stored independently, indexed by word positions in the sentence • Why do this? • Eliminate recalculation when tree branches are abandoned and later rebuilt • Concisely represent multiple parses

  13. Earley Parsing Example S → NP VP NP → ART N VP → V ADJ

  14. Earley Parsing Example S → NP VP NP → ART N VP → V ADJ

  15. Earley Parsing Example S → NP VP NP → ART N VP → V ADJ

  16. Earley Parsing Example S → NP VP NP → ART N VP → V ADJ

  17. Earley Parsing Example S → NP VP NP → ART N VP → V ADJ

  18. Earley Parsing Example S → NP VP NP → ART N VP → V ADJ

  19. Earley Parsing Example S → NP VP NP → ART N VP → V ADJ

  20. Earley Parsing Example S → NP VP NP → ART N VP → V ADJ

  21. Earley Parsing Example S → NP VP NP → ART N VP → V ADJ

  22. Probabilistic Parsing • How do we parse probabilistically? • Assign probabilities to grammar rules and words in lexicon • Grammar and lexicon “randomly” generate all possible sentences in the language • P(parse tree) = P(sentence generation)

  23. Probabilistic Parsing • Terminology • Earley state: each step of the processing that a constituent undergoes. Examples: • Starting sentence • Half-finished sentence • Complete sentence • Half-finished noun phrase • etc. • Earley path: a sequence of linked states • Example: the complete parse just described

  24. Probabilistic Parsing • Can represent the parse as a Markov chain: • Markov assumption (state probability is independent of path) applies, due to CFG NP ► ART NBegin NP Done NP Half Done S Half Done S ► NP VP Begin Scan “the” Scan “ball” Complete NP S Begin Predict NP etc. NP ► PNBegin Predict S (path abandoned)

  25. Probabilistic Parsing • Every Earley path corresponds to a parse tree • P(tree) = P(path) • Assign a probability to each state transition • Prediction: probability of grammar rule • Scanning: probability of word in lexicon • Completion: accumulated (“inner”) probability of the finished constituent • P(path) = product of P(transition)s

  26. Probabilistic Parse Example Grammar Lexicon

  27. Probabilistic Parse Example

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  29. Probabilistic Parse Example

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  37. Probabilistic Parse Example

  38. Prefix Probabilities • Current algorithm reports parse tree probability when the sentence is completed • What if we don’t have a full sentence? • Probability is tracked by constituent (“inner”), rather than by path (“forward”)

  39. Prefix Probabilities • Solution: add a separate path probability • Same as before, but propagate down on prediction step • This is the missing link to chain the path probabilities together

  40. Prefix Probability Example

  41. Prefix Probability Example

  42. Prefix Probability Example

  43. Prefix Probability Example

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  45. Prefix Probability Example

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  48. Prefix Probability Example

  49. Summary • Use Earley chart parser for efficient parsing, even with ambiguous or complex sentences • Use probabilities to choose among multiple possible parse trees • Track constituent probability for complete sentences • Also track path probability for incomplete sentences

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