The document discusses structured lexicons and lexical semantics, focusing on WordNet, a lexical database that organizes words by meaning rather than form. It describes the hierarchical organization of words into categories such as nouns, verbs, adjectives, and adverbs, and explores various lexical relations like synonymy and hyponymy. Additionally, the document outlines the applications of WordNet in areas such as document classification, word-sense disambiguation, and textual entailment.

1. Structured lexicons
and 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/WordNet
and 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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16.2 WORDNET: A Database of
Lexical Relations
Scope of current WordNet 1.6 release in terms
of unique entries and total numbers of senses
for the four databases.

13. 16.2 WORDNET: A Database of
Lexical Relations
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

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