Ling 570 Day 17: Named Entity Recognition Chunking.

Ling 570 Day 17:Named Entity Recognition

Chunking

Sequence Labeling

• Goal: Find most probable labeling of a sequence

• Many sequence labeling tasks– POS tagging– Word segmentation– Named entity tagging– Story/spoken sentence segmentation– Pitch accent detection– Dialog act tagging

NER AS SEQUENCE LABELING

NER as Classification Task

• Instance:


• Instance: token• Labels:


• Instance: token• Labels:– Position: B(eginning), I(nside), Outside


• Instance: token• Labels:– Position: B(eginning), I(nside), Outside– NER types: PER, ORG, LOC, NUM


• Instance: token• Labels:– Position: B(eginning), I(nside), Outside– NER types: PER, ORG, LOC, NUM– Label: Type-Position, e.g. PER-B, PER-I, O, …– How many tags?


• Instance: token• Labels:– Position: B(eginning), I(nside), Outside– NER types: PER, ORG, LOC, NUM– Label: Type-Position, e.g. PER-B, PER-I, O, …– How many tags?• (|NER Types|x 2) + 1

NER as Classification: Features

• What information can we use for NER?



– Predictive tokens: e.g. MD, Rev, Inc,..• How general are these features?



– Predictive tokens: e.g. MD, Rev, Inc,..• How general are these features? – Language? Genre? Domain?

NER as Classification: Shape Features

• Shape types:


• Shape types:– lower: e.g. e. e. cummings• All lower case



– capitalized: e.g. Washington• First letter uppercase




– all caps: e.g. WHO• all letters capitalized





– mixed case: eBay• Mixed upper and lower case


• Shape types:– lower: e.g. e. e. cummings

• All lower case




– Capitalized with period: H.



• All lower case




– Capitalized with period: H.– Ends with digit: A9



• All lower case




– Capitalized with period: H.– Ends with digit: A9– Contains hyphen: H-P

Example Instance Representation

• Example

Sequence Labeling

• Example

Evaluation

• System: output of automatic tagging• Gold Standard: true tags

Evaluation


• Precision: # correct chunks/# system chunks• Recall: # correct chunks/# gold chunks• F-measure:

Evaluation



• F1 balances precision & recall

Evaluation

• Standard measures:– Precision, Recall, F-measure– Computed on entity types (Co-NLL evaluation)

Evaluation


• Classifiers vs evaluation measures– Classifiers optimize tag accuracy

Evaluation


• Classifiers vs evaluation measures– Classifiers optimize tag accuracy• Most common tag?

Evaluation


• Classifiers vs evaluation measures– Classifiers optimize tag accuracy• Most common tag?

– O – most tokens aren’t NEs

– Evaluation measures focuses on NE

Evaluation


• Classifiers vs evaluation measures– Classifiers optimize tag accuracy

• Most common tag? – O – most tokens aren’t NEs

– Evaluation measures focuses on NE• State-of-the-art:– Standard tasks: PER, LOC: 0.92; ORG: 0.84

Hybrid Approaches

• Practical sytems– Exploit lists, rules, learning…

Hybrid Approaches

• Practical sytems– Exploit lists, rules, learning…– Multi-pass:• Early passes: high precision, low recall• Later passes: noisier sequence learning

Hybrid Approaches

• Practical sytems– Exploit lists, rules, learning…– Multi-pass:• Early passes: high precision, low recall• Later passes: noisier sequence learning

• Hybrid system:– High precision rules tag unambiguous mentions• Use string matching to capture substring matches

Hybrid Approaches

• Practical sytems– Exploit lists, rules, learning…– Multi-pass:

• Early passes: high precision, low recall• Later passes: noisier sequence learning

• Hybrid system:– High precision rules tag unambiguous mentions

• Use string matching to capture substring matches

– Tag items from domain-specific name lists– Apply sequence labeler

CHUNKING

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• [NPThe morning flight][PPfrom][NPDenver][Vphas arrived]

• [NPThe morning flight] from [NPDenver] has arrived

ExampleS

NP

NNP

Breaking

NNP

Dawn

VP

VBZ

has

VP

VBN

broken

PP

IN

into

NP

DT

the

NN

box

NN

office

NN

top

NN

ten

NPPPVPNP

ExampleS

NP

NNP

Breaking

NNP

Dawn

VP

VBZ

has

VP

VBN

broken

PP

IN

into

NP

DT

the

NN

box

NN

office

NN

top

NN

ten

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

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

Approaches

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

• Machine Learning– Similar to POS tagging

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


• 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


• 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?

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

Example

Chunking by Classification

• Model chunking as task similar to POS tagging• Instance:


• Model chunking as task similar to POS tagging• Instance: tokens • Labels: – Simultaneously encode segmentation &

identification



identification– IOB (or BIO tagging) (also BIOE or BIOSE)• Segment: B(eginning), I (nternal), O(utside)



identification– IOB (or BIO tagging) (also BIOE or BIOSE)• Segment: B(eginning), I (nternal), O(utside)• Identity: Phrase category: NP, VP, PP, etc.



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


• Model chunking as task similar to POS tagging• Instance: tokens • Labels: – Simultaneously encode segmentation & identification– IOB (or BIO tagging) (also BIOE, etc.)

• 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

Features for Chunking

• What are good features?

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

Chunking as Classification

• Example

Evaluation

• System: output of automatic tagging• Gold Standard: true tags – Typically extracted from parsed treebank


• F1 balances precision & recall

State-of-the-Art

• Base NP chunking: 0.96

State-of-the-Art

• Base NP chunking: 0.96• Complex phrases: Learning: 0.92-0.94

• Most learners achieve similar results

– Rule-based: 0.85-0.92

State-of-the-Art



– Rule-based: 0.85-0.92• Limiting factors:

State-of-the-Art



– Rule-based: 0.85-0.92• 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

Ling 570 Day 17: Named Entity Recognition Chunking.

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