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Sequence Classification: Chunking Shallow Processing Techniques for NLP Ling570 November 28, 2011.

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Presentation on theme: "Sequence Classification: Chunking Shallow Processing Techniques for NLP Ling570 November 28, 2011."— Presentation transcript:

1 Sequence Classification: Chunking Shallow Processing Techniques for NLP Ling570 November 28, 2011

2 Chunking

3 Roadmap Chunking Definition Motivation Challenges Approach

4 What is Chunking? Form of partial (shallow) parsing

5 What is Chunking? Form of partial (shallow) parsing Extracts major syntactic units, but not full parse trees

6 What is Chunking? Form of partial (shallow) parsing Extracts major syntactic units, but not full parse trees Task: identify and classify Flat, non-overlapping segments of a sentence

7 What is Chunking? Form of partial (shallow) parsing Extracts major syntactic units, but not full parse trees Task: identify and classify Flat, non-overlapping segments of a sentence Basic non-recursive phrases

8 What is Chunking? Form of partial (shallow) parsing Extracts major syntactic units, but not full parse trees Task: identify and classify Flat, non-overlapping segments of a sentence Basic non-recursive phrases Correspond to major POS May ignore some categories; i.e. base NP chunking

9 What is Chunking? Form of partial (shallow) parsing Extracts major syntactic units, but not full parse trees Task: identify and classify Flat, non-overlapping segments of a sentence Basic non-recursive phrases Correspond to major POS May ignore some categories; i.e. base NP chunking Create simple bracketing [ NP The morning flight][ PP from][ NP Denver][ Vp has arrived]

10 What is Chunking? Form of partial (shallow) parsing Extracts major syntactic units, but not full parse trees Task: identify and classify Flat, non-overlapping segments of a sentence Basic non-recursive phrases Correspond to major POS May ignore some categories; i.e. base NP chunking Create simple bracketing [ NP The morning flight][ PP from][ NP Denver][ Vp has arrived] [ NP The morning flight] from [ NP Denver] has arrived

11 Why Chunking? Used when full parse unnecessary

12 Why Chunking? Used when full parse unnecessary Or infeasible or impossible (when?)

13 Why Chunking? Used when full parse unnecessary Or infeasible or impossible (when?) Extraction of subcategorization frames Identify verb arguments e.g. VP NP VP NP NP VP NP to NP

14 Why Chunking? Used when full parse unnecessary Or infeasible or impossible (when?) Extraction of subcategorization frames Identify verb arguments e.g. VP NP VP NP NP VP NP to NP Information extraction: who did what to whom

15 Why Chunking? Used when full parse unnecessary Or infeasible or impossible (when?) Extraction of subcategorization frames Identify verb arguments e.g. VP NP VP NP NP VP NP to NP Information extraction: who did what to whom Summarization: Base information, remove mods

16 Why Chunking? Used when full parse unnecessary Or infeasible or impossible (when?) Extraction of subcategorization frames Identify verb arguments e.g. VP NP VP NP NP VP NP to NP Information extraction: who did what to whom Summarization: Base information, remove mods Information retrieval: Restrict indexing to base NPs

17 Processing Example Tokenization: The morning flight from Denver has arrived

18 Processing Example Tokenization: The morning flight from Denver has arrived POS tagging: DT JJ N PREP NNP AUX V

19 Processing Example Tokenization: The morning flight from Denver has arrived POS tagging: DT JJ N PREP NNP AUX V Chunking: NP PP NP VP

20 Processing Example Tokenization: The morning flight from Denver has arrived POS tagging: DT JJ N PREP NNP AUX V Chunking: NP PP NP VP Extraction: NP NP VP etc

21 Approaches Finite-state Approaches Grammatical rules in FSTs Cascade to produce more complex structure

22 Approaches Finite-state Approaches Grammatical rules in FSTs Cascade to produce more complex structure Machine Learning Similar to POS tagging

23 Finite-State Rule-Based Chunking Hand-crafted rules model phrases Typically application-specific

24 Finite-State Rule-Based Chunking Hand-crafted rules model phrases Typically application-specific Left-to-right longest match (Abney 1996) Start at beginning of sentence Find longest matching rule

25 Finite-State Rule-Based Chunking Hand-crafted rules model phrases Typically application-specific Left-to-right longest match (Abney 1996) Start at beginning of sentence Find longest matching rule Greedy approach, not guaranteed optimal

26 Finite-State Rule-Based Chunking Chunk rules: Cannot contain recursion NP -> Det Nominal:

27 Finite-State Rule-Based Chunking Chunk rules: Cannot contain recursion NP -> Det Nominal: Okay Nominal -> Nominal PP:

28 Finite-State Rule-Based Chunking Chunk rules: Cannot contain recursion NP -> Det Nominal: Okay Nominal -> Nominal PP: Not okay Examples: NP  (Det) Noun* Noun NP  Proper-Noun VP  Verb VP  Aux Verb

29 Finite-State Rule-Based Chunking Chunk rules: Cannot contain recursion NP -> Det Nominal: Okay Nominal -> Nominal PP: Not okay Examples: NP  (Det) Noun* Noun NP  Proper-Noun VP  Verb VP  Aux Verb Consider: Time flies like an arrow Is this what we want?

30 Cascading FSTs Richer partial parsing Pass output of FST to next FST

31 Cascading FSTs Richer partial parsing Pass output of FST to next FST Approach: First stage: Base phrase chunking Next stage: Larger constituents (e.g. PPs, VPs) Highest stage: Sentences

32 Example

33 Chunking by Classification Model chunking as task similar to POS tagging Instance:

34 Chunking by Classification Model chunking as task similar to POS tagging Instance: tokens Labels: Simultaneously encode segmentation & identification

35 Chunking by Classification Model chunking as task similar to POS tagging Instance: tokens Labels: Simultaneously encode segmentation & identification IOB (or BIO tagging) (also BIOE or BIOSE) Segment: B(eginning), I (nternal), O(utside)

36 Chunking by Classification Model chunking as task similar to POS tagging Instance: tokens Labels: Simultaneously encode segmentation & identification IOB (or BIO tagging) (also BIOE or BIOSE) Segment: B(eginning), I (nternal), O(utside) Identity: Phrase category: NP, VP, PP, etc.

37 Chunking by Classification Model chunking as task similar to POS tagging Instance: tokens Labels: Simultaneously encode segmentation & identification IOB (or BIO tagging) (also BIOE or BIOSE) Segment: B(eginning), I (nternal), O(utside) Identity: Phrase category: NP, VP, PP, etc. The morning flight from Denver has arrived NP-B NP-I NP-I PP-B NP-B VP-B VP-I

38 Chunking by Classification Model chunking as task similar to POS tagging Instance: tokens Labels: Simultaneously encode segmentation & identification IOB (or BIO tagging) (also BIOE or BIOSE) Segment: B(eginning), I (nternal), O(utside) Identity: Phrase category: NP, VP, PP, etc. The morning flight from Denver has arrived NP-B NP-I NP-I PP-B NP-B VP-B VP-I NP-B NP-I NP-I NP-B

39 Features for Chunking What are good features?

40 Features for Chunking What are good features? Preceding tags for 2 preceding words

41 Features for Chunking What are good features? Preceding tags for 2 preceding words Words for 2 preceding, current, 2 following

42 Features for Chunking What are good features? Preceding tags for 2 preceding words Words for 2 preceding, current, 2 following Parts of speech for 2 preceding, current, 2 following

43 Features for Chunking What are good features? Preceding tags for 2 preceding words Words for 2 preceding, current, 2 following Parts of speech for 2 preceding, current, 2 following Vector includes those features + true label

44 Chunking as Classification Example

45 Evaluation System: output of automatic tagging Gold Standard: true tags Typically extracted from parsed treebank Precision: # correct chunks/# system chunks Recall: # correct chunks/# gold chunks F-measure: F 1 balances precision & recall

46 State-of-the-Art Base NP chunking: 0.96

47 State-of-the-Art Base NP chunking: 0.96 Complex phrases: Learning: Most learners achieve similar results Rule-based:

48 State-of-the-Art Base NP chunking: 0.96 Complex phrases: Learning: Most learners achieve similar results Rule-based: Limiting factors:

49 State-of-the-Art Base NP chunking: 0.96 Complex phrases: Learning: Most learners achieve similar results Rule-based: Limiting factors: POS tagging accuracy Inconsistent labeling (parse tree extraction) Conjunctions Late departures and arrivals are common in winter Late departures and cancellations are common in winter


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