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A Pipeline Model for Bottom-Up Dependency Parsing

A Pipeline Model for Bottom-Up Dependency Parsing. Tenth Conference on Natural Language Learning, Shared Task New York, USA, 2006. Ming-Wei Chang, Quang Do , Dan Roth Computer Science Department University of Illinois, Urbana-Champaign. Dependency Parsing Approach: Summary.

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A Pipeline Model for Bottom-Up Dependency Parsing

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  1. A Pipeline Model for Bottom-Up Dependency Parsing Tenth Conference on Natural Language Learning, Shared Task New York, USA, 2006 Ming-Wei Chang, Quang Do, Dan Roth Computer Science Department University of Illinois, Urbana-Champaign

  2. Dependency Parsing Approach: Summary • Modified Shift-Reduced parser. • Actions are selected via a classifier + Extended action set + Look ahead search • Control Policy: Left to right, with step back • Dependency Types: A separate multiclass classifier • Multilingual: Convert non-projective languages to projective [Nirve and Nilsson, 2005]

  3. Motivation • Shift-Reduced Parsing as a pipeline model: • A classifier is used to determine which action to take • The decision at each stage depends on previous decisions. + Making a decision can rely on information acquired in previous stages - Making a decision can rely on incorrect information acquired in previous stages. • Viewed this way, we want to: • Reduce the number of decisions • Make local decisions more robust

  4. S L R x a b a b a WL b c a b a b c d a b d c Parsing Algorithm • Parsing from left to right; considering pair of (currently) consecutive words (a,b) (with a<b) • For the pair (a, b), to become the child of a, b must be a complete subtree. • Standard action set: Left, Right, Shift [Yamada and Matsumoto, 03], • Left: a is the parent of b • Right: b is the parent of a • Shift: the action is not Left or Right. +We split Shiftinto Shift, WaitLeft,WaitRight • WaitLeft: a is the parent of b, but b is the parent of other nodes. Action is deferred. • WaitRight (?): not needed ! • Control policy: Step Back • Provably, allows parsing in one pass over the sentence [ACL06] • Reduces the number of decisions

  5. w x y z w y z x A Pipeline Model with Look Ahead Search • Pipilining decisions may result in error accumulation: • The correct dependencies  • If the algorithm decides w  x before xy and x  z, we cannot recover the correct parent for y and z. • Correct early decisions are crucial • A look ahead search algorithm takes into account future predicted actions • Local decisions are more robust

  6. keep this action keep this action keep this action a0 a1 a0 a1 a2 a0 a1 a2 a3 depth=1 depth=2 depth=3 A Pipeline Model with Look Ahead – cont’ • The search algorithm performs a search of length depth. • Additive scoring is used to score the sequence • The first action in this sequence is performed.

  7. Experiments (for Swedish)* • The effect of the new action. • The effect of look ahead search. *For other languages, please refer to our paper.

  8. Analysis • WaitLeft and the Look Ahead Search improve the parsing results. Results can be improved by: • Selecting features and parameters more carefully • Currently we use exactly the same set of features and the same parameters for all languages. • Using the FEAT column properly • The result for languages with FEAT column is generally worse than the languages without FEAT column

  9. Thank you !

  10. It is not tractable to find the global optimal predicted sequence in the pipeline model with the large depth. • In the pipeline framework, the feature vector of current decision depends on every previous predictions. • The FEAT column • Average difference between our system with the best systems • With Feats 4.5%, Without Feats 3.4%

  11. Labeling the Dependency Type • A post-task after predicting the head for the tokens in the sentence. • This is a multi-class classification. • Consider every edge of the tree • Classify the edge into several classes • The parents of the tokens which were labeled in the first phase will be used as the features.

  12. x L a b a b R a b WL a b c

  13. Experiments • We show the effect of the new action (on Swedish).

  14. Experiments • The effect of look ahead search (on Swedish).

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