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CPSC 503 Computational Linguistics

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bank commercial bank1 vs. river bank2 ... (hold back to a later time; 'let's postpone the exam') 8/23/09. CPSC503 Spring 2004 ... – PowerPoint PPT presentation

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Title: CPSC 503 Computational Linguistics


1
CPSC 503Computational Linguistics
  • Lexical Semantics
  • Lecture 17
  • Giuseppe Carenini

2
Semantic Analysis
Sentence
Meanings of grammatical structures
  • Syntax-driven
  • Semantic Analysis

Meanings of words
Literal Meaning
I N F E R E N C E
Common-Sense Domain knowledge
Further Analysis
Discourse Structure
Intended meaning
Context
3
Word Meaning in Syntax-driven SA
assigning constants
  • Attachments
  • AyCaramba
  • MEAT
  • PropNoun -gt AyCaramba
  • MassNoun -gt meat

4
Today 19/3
  • How much it is missed by this narrow view!
  • Relations among words and their meanings
  • Internal structure of individual words

5
Word Meaning Theory
  • Paradigmatic the external relational structure
    among words
  • Syntagmatic the internal structure of words that
    determines where they can go and what they can do

6
Word?
  • Lexeme
  • Orthographic form
  • Phonological form
  • symbolic Meaning representation (sense)

content?
duck?
bank?
  • Lexicon A collection of lexemes

7
Dictionary
  • Repositories of information about the meaning of
    words, but..

Most of the definitions are circular ?? They are
descriptions.
Fortunately, there is still some useful semantic
info (Lexical Relations) L1,L2 same O/P,
different M L1,L2 same M, different O L1,L2
opposite M L1,L2 , M1 subclass of M1
Homonymy
Synonymy
Antonymy
Hyponymy
8
Homonymy
  • Def. Lexemes that have the same form but
    unrelated meanings
  • Examples Bat (wooden stick-like thing) vs
    Bat (flying scary mammal thing)

Plant (.) vs Plant ()
9
Homonymy NLP Tasks
  • Information retrieval
  • QUERY bat
  • Spelling correction homophones can lead to
    real-word spelling errors
  • Text-to-Speech Homographs (which are not
    homophones)

10
Polysemy
  • Def. The case where a single lexeme has multiple
    related meanings.

Consider the homonym bank ? commercial bank1
vs. river bank2
  • Now consider A PCFG can be trained using
    derivation trees from a tree bank annotated by
    human experts
  • Is this a new independent sense of bank?

11
Polysemy
  • Lexeme (new def.)
  • Orthographic form Phonological form
  • Set of related senses

How many distinct (but related) senses?
  • They serve meat
  • He served as Dept. Head
  • She served her time.

Different subcat
Intuition (prison)
  • Does AC serve vegetarian food?
  • Does AC serve Rome?
  • Does AC serve vegetarian food and Rome?

Zeugma
12
Synonyms
  • Def. Different lexemes with the same meaning.
  • Substitutability- if they can be substituted for
    one another in some environment without changing
    meaning or acceptability.
  • Would I be flying on a large/big plane?

? became kind of a large/big sister to
? You made a large/big mistake
13
Hyponymy
  • Def. Pairings where one lexeme denotes a subclass
    of the other
  • Since dogs are canids
  • Dog is a hyponym of canid and
  • Canid is a hypernym of dog
  • car/vehicle
  • doctor/human

14
Lexical Resources
  • Databases containing all lexical relations among
    all lexemes
  • Development
  • Mining info from dictionaries and thesauri
  • Handcrafting it from scratch
  • WordNet most well-developed and widely used
    Fellbaum 1998
  • for English (versions for other languages are
    under development)

15
WordNet
  • Probably numbers out of date
  • For each lexeme (no phonological distinctions)
    all possible senses (no distinction between
    homonymy and polysemy)
  • For each sense a set of synonyms (synset) and a
    gloss

16
WordNet table entry
  • The noun "table" has 6 senses in WordNet.1.
    table, tabular array -- (a set of data )2.
    table -- (a piece of furniture )3. table -- (a
    piece of furniture )4. mesa, table -- (flat
    tableland )5. table -- (a company of people
    )6. board, table -- (food or meals )

The verb "table" has 1 sense in WordNet.1.
postpone, prorogue, hold over, put over, table,
shelve, set back, defer, remit, put off (hold
back to a later time "let's postpone the exam")
17
WordNet Relations
fi
18
WordNet Hierarchies
  • WordNet example from ver1.7.1
  • Sense 3 Vancouver
  • ?city, metropolis, urban center
  • ? municipality
  • ? urban area
  • ? geographical area
  • ? region
  • ? location
  • ? entity, physical thing
  • ? administrative district, territorial division
  • ? district, territory
  • ? region
  • ? location ? entity, physical thing
  • ? port
  • ? geographic point
  • ? point
  • ? location
  • ? entity, physical thing

19
Wordnet NLP Tasks
  • Probabilistic Parsing (PP-attachments) words
    word-classes extracted from the hypernym
    hierarchy increase accuracy from 84 to 88
    Stetina and Nagao, 199?7-8?
  • Express Selectional Preferences for verbs
  • Word sense disambiguation (next class)
  • Information retrieval (next class)
  • Lexical Chains (summarization)

20
Today Outline
  • How much it is missed by this narrow view!
  • Relations among words and their meanings
  • Internal structure of individual words

21
Predicate-Argument Structure
  • Represent relationships among concepts
  • Some words act like arguments and some words act
    like predicates
  • Nouns as concepts or arguments red(ball)
  • Adj, Adv, Verbs as predicates red(ball)

I ate a turkey sandwich for lunch w
Isa(w,Eating) Ù Eater(w,Speaker) Ù
Eaten(w,TurkeySandwich) Ù MealEaten(w,Lunch)
AyCaramba serves meat w Isa(w,Serving) Ù
Server(w,Speaker) Ù Served(w,Meat)
22
Thematic Roles
  • Def. Semantic generalizations over the specific
    roles that occur with specific verbs.
  • I.e. eaters, servers, takers, givers, makers,
    doers, killers, all have something in common
  • We can generalize (or try to) across other roles
    as well

23
Thematic Roles
  • Not definitive, not from a single theory!

24
Thematic Role Examples
fl
25
Thematic Roles Usage
Sentence
Syntax-driven Semantic Analysis
Literal Meaning expressed with thematic roles
Further Analysis
Eg. Result did not exist before
Intended meaning
26
Thematic Roles Resources
  • Databases containing for each verb its syntactic
    and thematic argument structures
  • Development
  • Corpus-based
  • Handcrafting it from scratch
  • FrameNet most well-developed (book online
    Version 1.1 Printed December 30, 2003)
  • for English (versions for other languages are
    under development)

27
FrameNet Entry
  • Hiring
  • Definition An Employer hires an Employee,
    promising the Employee a certain Compensation in
    exchange for the performance of a job. The job
    may be described either in terms of a Task or a
    Position in a Field.
  • Inherits From Intentionally affect
  • Lexical Units commission.v, hire.n, hire.v,
    retain.v, sign.v, take on.v

28
FrameNet Annotations
Some roles..
Employer
Employee
Task
Position
  • np-vpto
  • In 1979 , singer Nancy Wilson HIRED him to open
    her nightclub act .
  • .
  • np-ppas
  • Castro has swallowed his doubts and HIRED
    Valenzuela as a cook in his small restaurant .

Includes counting How many times a role was
expressed with a particular syntactic structure
29
Summary
  • Relations among words and their meanings

Wordnet
  • Internal structure of individual words

FrameNet
30
Next Time
  • Word Sense disambiguation
  • and Information Retrieval
  • Read Chp. 17
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