wordnet: a database of lexical relations

Structured lexiconsand Lexical semantics

Especially WordNet®

See D Jurafsky & JH Martin: Speech and Language Processing, Upper Saddle River NJ (2000): Prentice Hall, Chapter 16.

and http://en.wikipedia.org/wiki/WordNetand explore WordNet: http://wordnet.princeton.edu/

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Structured lexicons• Alternative to alphabetical dictionary• List of words grouped according to meaning• Classic example Roget’s Thesaurus• Hierarchical organization is important• Hierarchies familiar as taxonomies, eg in natural

sciences– Daughters are “types of” and share certain properties,

inherited from the mother• Similar idea for ordinary words: hyponymy and

synonymy

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animal

bird fish ...

canary eagle trout shark

bald e. golden e. hawk e. bateleur

space

in general dimensions form motion

size expansion distance interval contiguity

reduction, deflation, shrinkage, curtailment, condensation ....

hyponymy

synonymy

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Thesaurus• A way to show the structure of (lexical)

knowledge• Much used for technical terminology• Can be enriched by having other lexical

relations:– Antonyms (as well as synonyms)– Different hyponymy relations, not just is-a-type-of, but

has-as-part/member• Thesaurus can be explored in any direction

– across, up, down– Some obvious distance metrics can be used to

measure similarity between words

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WordNet: History

• 1985: a group of psychologists and linguists start to develop a “lexical database”– Princeton University– theoretical basis: results from

• WordNet organizes lexical information in terms of word meanings, rather than word forms.

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Global organisation

• division of the lexicon into five categories:– Nouns– Verbs– Adjectives– Adverbs– function words (“probably stored separately

as part of the syntactic component of language” [Miller et al.]

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Global organization

• nouns: organized as topical hierarchies• verbs: entailment relations• adjectives: multi-dimensional hyperspaces• adverbs: multi-dimensional hyperspaces

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Lexical semantics• How are word meanings represented in WordNet?

– synsets (synonym sets) as basic units– a word ‘meaning’ is represented by simply listing the word forms

that can be used to express it• example: senses of board

– a piece of lumber vs. a group of people assembled for some purpose

– synsets as unambiguous designators:– {board, plank, ...} vs. {board, committee, ...}

• Members of synsets are rarely true synonyms– WordNet does not attempt to capture subtle distinctions among

members of the synset– may be due to specific details, or simply connotation, collocation

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Synsets

• synsets often sufficient for differential purposes

• Synsets are linked by semantic relations, word forms are linked by lexical relations..

• Preferable for cardinality of synset to be >1– WordNet also gives a gloss for each word

meaning, and (often) an example

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16.2 WORDNET: A Database of Lexical Relations

• WordNet:– The most well-developed and widely used

lexical DB for English– Handcrafting from scratch, rather than

mining information from existing dictionaries and thesauri

– Consisting three separate DBs:• One each for nouns and verbs, and• A third for adjectives and adverbs

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Scope of current WordNet 1.6 release in terms of unique entries and total numbers of senses for the four databases.


A portion of the WordNet 1.6 entry for the noun bass

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Lexical relations in WordNet

• WordNet is organized by semantic relations.– It is characteristic of semantic relations that they are

reciprocated– if there is a semantic relation R between meaning {x1,

x2, ...} and meaning {y1, y2, ...}, then there is a relation R between {y1,y2, ...} and {x1, x2, ...}

– Individual relations may or may not be• Symmetric R(A,B) R(B,A) (eg synonymy, not hyponymy)• Transitive R(A,B) & R(B,C) R(A,C) (eg synonymy may be)• Reflexive R(A,A) is true (synonymy is, antonymy isn’t)

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Lexical relations• Nouns

– Synonym ~ antonym (opposite of)– Hypernyms (is a kind of) ~ hyponym (for example)– Holonym (is part of) ~ meronym (has as part)

• Verbs– Synonym ~ antonym– Hypernym ~ troponym (eg lisp – talk) – Entailment (eg snore – sleep)

• Adjectives/Adverbs in addition to above– Related nouns– Verb participles– Derivational information

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Lexical relations

Noun relations in WordNet

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Lexical relations

Verb relations in WordNet

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Lexical relations

Adjective and adverb relations in WordNet

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WordNet’s noun hierarchy

• noun hierarchy partitioned into separate hierarchies with unique top hypernyms

• vague abstractions would be semantically empty, e.g. {entity} with immediate hyponyms {object, thing} and {idea}

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• {act,action,activity}• {animal,fauna}• {artifact}• {attribute,property}• {body,corpus}• {cognition,knowledge}• {communication}• {event,happening}• {feeling,emotion}• {food}• {group,collection}• {location,place}• {motive}

• {natural object}• {natural phenomenon}• {person,human being}• {plant,flora}• {possession}• {process}• {quantity,amount}• {relation}• {shape}• {state,condition}• {substance}• {time}

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Nouns in WordNet

• noun hierarchy as lexical inheritance system– seldom goes more than ten levels deep, – the deepest examples usually contain

technical levels that are not part of everyday vocabulary

– shallowest levels are too vague– “Inherited hypernym” option shows full

hierarchy

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deep

shallow

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Nouns in WordNet

• man-made artefacts: sometimes six or seven levels deep– roadster → car → motor vehicle → wheeled vehicle

→ vehicle → conveyance → artefact• hierarchy of persons: about three or four levels

– televangelist → evangelist → preacher → clergyman → spiritual leader → person

• Like all thesaurus structures, words can have multiple hypernyms

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WordNets for other languages

• Idea has been widely copied• Sometimes by “translating” Princeton WordNet

– Lexical relations in general are universal ...– But are they in practice?– Are synsets universal?

• EuroWordNet: combining multilingual WordNets to include cross-language equivalence– Inherent difficulties, as above

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What can WordNet be used for?

• As a lexical resource, an online dictionary, for human use

• Word-sense disambiguation (including homophone correction)– neighbouring words will be more closely

related to correct sense (desert/dessert ~ camel)

• Document classification– What is this text about? Look for recurring

hypernyms

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What can WordNet be used for?

• Document retrieval– eg looking for texts about sports cars, search

for synonyms and hyponyms of sports car• Open-domain Q/A

– Searching texts (eg WWW) to answer questions expressed in natural language

– eg http://uk.ask.com/ [example]• Textual entailment

– Answering questions implied by text

wordnet: a database of lexical relations

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