Using resources

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Using resources

• WordNet and the BNC

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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
• psycholinguistics and psycholexicology
• What are properties of the mental lexicon?

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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 N-dimensional hyperspaces
• adverbs N-dimensional hyperspaces
• Miller et al. Each of these lexical
  structures reflects a different way of
  categorizing experience attempts to impose a
  single organizing principle on all syntactic
  categories would badly misrepresent the
  psychological complexity of lexical knowledge.

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Basic principles

• organize lexical information in terms of word
  meaning, rather than word forms
• In this respect, WordNet resembles athesaurus
  more than a dictionary, ... Miller et al.
• ... a word is a conventional association
  between a lexicalized concept and an utterance
  that plays a syntactic role.
• word form refers to physical utterance or
  inscription
• word meaning refers to the lexicalized concept
  that a form can be used to express

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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

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Synsets

• synsets often sufficient for differential
  purposes
• if an appropriate synonym is not available a
  short gloss may be used
• e.g. board, (a persons meals, provided
  regularly for money)

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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
  x, x, ... and meaning y, y, ..., then there
  is a relation R between y,y, ... and x, x,
  ....

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Lexical relations synonymy

• similarity of meaning
• Leibniz two expressions are synonymous if the
  substitution of one for the other never changes
  the truth value of a sentence in which the
  substitution is made
• such global synonymy is rare (it would be
  redundant)
• synonymy relative to a context two expressions
  are synonymous in a linguistic context C if the
  substitution of one for the other in C does not
  alter the truth value
• consequence of this synonymy in terms of
  substitutability words in different syntactic
  categories cannot be synonyms

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Lexical relations antonymy

• antonym of a word x is sometimes not-x, but not
  always
• rich and poor are antonyms
• but not rich does not imply poor
• (because many people consider them neither rich
  nor poor)
• antonymy is a lexical relation between word
  forms, not a semantic relation between word
  meanings
• meanings rise,ascend and fall, descend are
  conceptual opposites, but they are not antonyms
  rise/fall and ascend/descend are pairs of
  antonyms
• w1 w2? S1 w3 w4 ? S2 ant(w1 ,w3 ) ?
  ant(w2 ,w4 )

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Lexcial relations hyponymy

• hyponymy is a semantic relation between word
  meanings
• maple is a hyponym of tree
• inverse hypernymy
• tree is a hypernym of maple
• also called subordination/superordination
  subset/superset ISA relation
• test for hyponomy
• native speaker must accept sentences built from
  the frame An x is a (kind of) y

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Lexcial relations meronymy

• A concept represented by the synset x, x,...
  is a meronym of a concept represented by the
  synset y, y, ... if native speakers of English
  accept sentences constructed from such frames as
  A y has an x (as a part), An x is a part of
  y.
• inverse relation holonymy
• HAS-AS-PART
• part hierarchy
• part-of is asymmetric and (with caution)
  transitive

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Lexical relations meronymy

• failures of transitivity caused by different
  part-whole relations, e.g.
• A musician has an arm.
• An orchestra has a musician.
• but ? An orchestra has an arm.
• Types of meronymy in WordNet
• component most frequently found
• member
• composition
• phase process

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WordNets 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,ammount
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, and
  the deepest examples usually contain technical
  levels that are not part of everyday vocabulary.
• Shetland pony ? pony ? horse ? equid ? odd-toed
  ungulate ? herbivore ? mammal ? vertebrate ?
  animal

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Nouns in WordNet

• man-made artifacts sometimes six or seven levels
  deep
• roadster ? car ? motor vehicle ? wheeled
  vehicle ? vehicle ? conveyance ? artifact
• 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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BNC

• One of the most widely used corpora (esp. in
  Britain, but also elsewhere)
• A balanced synchronic text corpus containing 100
  million words (POS tagged)
• Collected in late 1980s
• 90 text, 10 transcribed speech
• Encoded according to TEI standards
• Associated tools (mainly for searching), but many
  users write their own (eg in Perl)
• http//www.natcorp.ox.ac.uk/

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Using the BNC

• Just looking up words
• More interesting to construct queries that
  exploit the mark-up (see Allans slides)
• Already becoming dated (e.g. numpty)
• Results often contradict authorities such as
  dictionaries, especially in revealing primary
  senses/uses of words.

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WWW as a corpus

• Standard Google search engine used with
  individual words does not always give good word
  collocations after all, Google is document
  retrieval
• Try http//labs1.google.com/sets

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Lexical research

• Use corpus resource such as BNc together with
  WordNet to get interesting results
• ? Allans slides