a classifier-based deterministic parser for chinese

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A Classifier-based Deterministic Parser for Chinese

-- Mengqiu Wang

Advisor: Prof. Teruko Mitamura Joint work with Kenji Sagae

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Outline of the talk

BackgroundDeterministic parsing modelClassifier and feature selectionPOS taggingExperiment and resultsDiscussion and future workConclusion

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Background

Constituency parsing is one of the most fundamental tasks in NLP.

State-of-the-art accuracy in Chinese constituency parsing achieves precision and recall in the lower 80% using automatically generated POS.

Most literature in parsing only reports accuracy, efficiency is typically ignored

But in reality, parsers are deemed too slow for many NLP applications (e.g. IR)

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Deterministic Parsing Model

Originally developed in [Sagae and Lavie 2005] Input

Convention in deterministic parsing assumes input sentences (Chinese in our case) are already segmented and POS tagged1.

Main Data Structure A queue, to store input word-POS pairs A stack, holds partial parse trees

Trees are lexicalized. We used the same head-finding rules as [Bikel 2004]

The Parser performs binary Shift-Reduce actions based on classifier decisions.

Example …

1. We perform our own POS tagging based on SVM

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Deterministic Parsing Model Cont.

Input sentence:

布朗 /NR (Brown/Proper Noun) 访问 /VV (Visits/Verb) 上海 /NR (Shanghai/Proper Noun)

Initial parser state:

Stack: ΘQueue: NR

布朗

VV

访问

NR

上海

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Action 1: Shift Parser State:

Stack:

Queue:

NR

布朗

VV

访问

NR

上海

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Action 2: Reduce the first item on stack to a NP node, with node (NR 布朗 ) as the head

Parser State:

Stack:

Queue: VV

访问

NR

上海

NR

布朗

NP (NR 布朗 )

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Stack:

Queue:

VV

访问

NR

上海

NR

布朗

NP (NR 布朗 )

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Stack:

Queue: Θ

VV

访问

NR

上海NR

布朗

NP (NR 布朗 )

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Action 5: Reduce the first item on stack to a NP node, with node (NR 上海 ) as the head

Parser State:

Stack:

Queue: Θ

VV

访问NR

布朗

NP (NR 布朗 )

NR

上海

NP (NR 上海 )

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Action 6: Reduce the first two items on stack to a VP node, with node (VV 访问 ) as the head

Parser State:

Stack:

Queue: Θ

NR

布朗

NP (NR 布朗 )

VV

访问 NR

上海

NP (NR 上海 )

VP (VV 访问 )

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Action 7: Reduce the first two items on stack to an IP node, take the head node of the VP subtree as the head -- (VV 访问 ).

Parser State:

Stack:

Queue: Θ

NR

布朗

NP (NR 布朗 )

VV

访问 NR

上海

NP (NR 上海 )

VP (VV 访问 )

VP (VV 访问 )

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Parsing terminates when queue is empty and stack only contains one item

Final parse tree:

NP (NR 上海 )

NR

布朗

NP (NR 布朗 )

VV

访问 NR

上海

VP (VV 访问 )

VP (VV 访问 )

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Classifiers

Classification is the most important part of deterministic parsing.

We experimented with four different classifiers: SVM classifier

finds a hyper-plane that gives the maximum soft margin that minimizes the expected risk.

Maximum Entropy Classifierestimates a set of parameters that would maximize the entropy over distributions that satisfy certain constraints which force the model to best account for the training data.

Decision Tree ClassifierWe used C4.5

Memory-based LearningkNN classifier, Lazy learner, short training time, ideal for prototyping.

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Features

The features we used are distributionally derived or linguistically motivated.

Each feature carries information about the context of a particular parse state.

We denote the top item on the stack as S(1), and second item (from the top) on the stack as S(2), and so on. Similarly, we denote the first item on the queue as Q(1), the second as Q(2), and so on.

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Features

A Boolean feature indicates if a closing punctuation is expected or not. A Boolean value indicates if the queue is empty or not. A Boolean feature indicates whether there is a comma separating S(1) and S(2) or

not. Last action given by the classifier, and number of words in S(1) and S(2). Headword and its POS of S(1), S(2), S(3) and S(4), and word and POS of Q(1),

Q(2), Q(3) and Q(4). Nonterminal label of the root of S(1) and S(2), and number of punctuations in S(1)

and S(2). Rhythmic features and the linear distance between the head-words of the S(1)

and S(2). Number of words found so far to be dependents of the head-words of S(1) and

S(2). Nonterminal label, POS and headword of the immediate left and right child of the

root of S(1) and S(2). Most recently found word and POS pair that is to the left of the head-word of S(1)

and S(2). Most recently found word and POS pair that is to the right of the head-word of S(1)

and S(2).

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POS tagging

In our model, POS tagging is treated as a separate problem and is done prior to parsing.

But we care about the performance of the parser in realistic situations with automatically generated POS tags.

We implemented a simple 2-pass POS tagging model based on SVM, achieved 92.5% accuracy.

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Experiments

Standard data collection Training set: section 1-270 of the Penn Chinese Treebank

(3484 sentences, 84873 words). Development set: section 301-326 Testing set: section 271-300 Total: 99629 words, about 1/10 of the size of English Penn

Treebank. Standard corpus preparation

Empty nodes were removed Functional label of nonterminal nodes removed.

Eg. NP-Subj -> NP For scoring we used the evalb1 program. Labeled

recall, labeled precision and F1 (harmonic mean) measures are reported.

1. http://nlp.cs.nyu.edu/evalb

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Results

Comparison of classifiers on development set using gold-standard POS

classification Parsing Accuracy

Model Accuracy LR LP F1 Fail Time

SVM 94.3% 86.9% 87.9% 87.4% 0 3m 19s

Maxent 92.6% 84.1% 85.2% 84.6% 5 0m 21s

DTree1 92.0% 78.8% 80.3% 79.5% 42 0m 12s

DTree2 N/A 81.6% 83.6% 82.6% 30 0m 18s

MBL 90.6% 74.3% 75.2% 74.7% 2 16m 11s

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Classifier Ensemble

Using stacked-classifier techniques, we improved the performance on the dev set to 90.3% LR and LP of 90.5%, which is a 3.4% improvement in LR and a 2.6% improvement in LP over the SVM model.

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Comparison with related work

Results on test set using automatically generated POS.

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Comparison with related work cont.

Comparison of parsing speed

Model Runtime

Bikel 54m 6s

Levy & Manning 8m 12s

DTree 0m 14s

Maxent 0m 24s

SVM 3m 50s

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Discussion and future work

Among the classifiers, SVM has high accuracy but low speed; DTree has lower accuracy but great speed; Maxent sits in between these two in terms of accuracy and speed.

It is desirable to bring the two ends of the spectrum closer, ie. increase the accuracy of DTree classifier, lower the computational cost of SVM classification.

Action items Apply boosting techniques (Adaboost, random forest,

bagging, etc.) to DTree. (Preliminary attempt didn’t yield better performance, calls for further investigation).

Feature selection (especially on lexical items) to reduce computational cost of classification

Re-implement the parser in C++ (avoid invoking external processes and expensive I/O

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Conclusion

Implemented a classifier based deterministic constituency parser for Chinese

We achieved comparable results to the state-of-the-art in Chinese parsing

Very fast parsing is made possible for applications that are speed-critical with some tradeoff in accuracy.

Advances in machine learning techniques can be directly applied to parsing problem, opens up lots of opportunities for further improvement

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Reference

Daniel M. Bikel and David Chiang. 2000. Two statistical parsing models applied to the Chinese Treebank. In Proceedings of the Second Chinese Language Processing Workshop.

Daniel M. Bikel. 2004. On the Parameter Space of Generative Lexicalized Statistical Parsing Models. Ph.D. thesis, University of Pennsylvania.

David Chiang and Daniel M. Bikel. 2002. Recovering latent information in treebanks. In Proceedings of the 19th International Conference on Computational Linguistics.

Michael John Collins. 1999. Head-driven Statistical Models for Natural Langauge Parsing. Ph.D. thesis, University of Pennsylvania.

Walter Daelemans, Jakub Zavrel, Ko van der Sloot, and Antal van den Bosch. 2004. Timbl: Tilburgmemory based learner, version 5.1, reference guide. Technical Report 04-02, ILK Research Group, Tilburg University.

Pascale Fung, Grace Ngai, Yongsheng Yang, and Benfeng Chen. 2004. A maximum-entropy Chinese parser augmented by transformation-based learning. ACM Transactions on Asian Language Information Processing, 3(2):159–168.

Mary Hearne and Andy Way. 2004. Data-oriented parsing and the Penn Chinese Treebank. In Proceedings of the First International Joint Conference on Natural Language Processing.

Zhengping Jiang. 2004. Statistical Chinese parsing. Honours thesis, National University of Singapore. Zhang Le, 2004. Maximum Entropy Modeling Toolkit for Python and C++. Reference Manual. Roger Levy and Christopher D. Manning. 2003. Is it harder to parse Chinese, or the Chinese Treebank? In

Proceedings of the 41st Annual Meeting of the Association for Computational Linguistics. Xiaoqiang Luo. 2003. A maximum entropy Chinese character-based parser. In Proceedings of the 2003

Conference on Empirical Methods in Natural Language Processing. David M. Magerman. 1994. Natural Language Parsing as Statistical Pattern Recognition. Ph.D. thesis,

Stanford University. Kenji Sagae and Alon Lavie. 2005. A classifier-based parser with linear run-time complexity. In Proceedings

of the Ninth International Workshop on Parsing Technology. Deyi Xiong, Shuanglong Li, Qun Liu, Shouxun Lin, and Yueliang Qian. 2005. Parsing the Penn Chinese

Treebank with semantic knowledge. In International Joint Conference on Natural Language Processing 2005.

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Thank you!

Questions?

a classifier-based deterministic parser for chinese

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