CS 904: Natural Language Processing LINGUISTIC ESSENTIALS

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CS 904 Natural Language ProcessingLINGUISTIC
ESSENTIALS

• L. Venkata Subramaniam
• January 10, 2002

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Parts of Speech

• Correspond to syntactic or grammatical categories
  such as noun, verb, adjectives, prepositions.
• Word categories are systematically related by
  morphological processes such as the formation of
  plural form from the singular form, past tense
  from present tense.

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The Parts of Speech

• Noun Refer to entities like people, places,
  things or idea.
• Pronoun words that take the place of nouns.
• Proper noun names.
• Determiner describes the particular action in a
  noun.
• Adjective describes the properties of nouns or
  pronouns.
• Verb action in a sentence.
• Adverb describes a verb, an adjective or
  another adverb.
• And many more

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POS Labeling

• Children (NOUN) eat (VERB) sweet(ADJECTIVE)
  candy(NOUN)
• The(ARTICLE) children(NOUN) ate(VERB)
  the(ARTICLE) cake(NOUN)
• The(ARTICLE) news(NOUN) has(AUXILIARY)
  been(MAIN VERB) quite(ADVERB)
  sad(ADJECTIVE) in(PREPOSITION) fact(NOUN)
  .(PERIOD)

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A Parse Tree
S
NP VP AT
NNS VBD NP The
children ate AT NN

the cake
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Context Free Grammars

• Grammars consisting entirely of rules with a
  single symbol on the left-hand side, called
  mother.
• Independent of previous or subsequent labels.

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A Simple Context-Free Grammar

• The Grammar rules
• S -gt NP V
• NP -gt N
• The Lexicon
• N -gt John, Gaurav, Ram
• V -gt walks, talks, eats, went ..

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Tag Sets

• A tag indicates the various conventional parts of
  speech.
• Different Tag Sets have been used E.g., Brown
  Tag Set, Penn Treebank Tag Set.
• Tag examples NP Proper noun, NN Singular noun,
  AT Article, DET Determinant.

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Stochastic Grammars

• Grammars obtained by adding probabilities in a
  fairly transparent way to algebraic (i. e.,
  non-probabilistic) grammars.
• Stochastic grammars supplement underlying
  algebraic grammars.

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Dependencies

• Local Dependency dependence between two words
  expressed within the same syntactic rule.
  (n-grams model this well)
• Non-local dependency is an instance in which two
  words can be syntactically dependent even though
  they occur far apart in a sentence.

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Ambiguities

• Children eat sweet candy
• Too much boiling will candy the molasses
• In sentence (1) candy is a noun while in (2) it
  is an adjective.
• Word category (POS) ambiguity needs to be
  resolved.

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Ambiguities (Cont.)

• Semantic Roles Determining thematic roles in a
  sentence.
• Agent, Patient, Experiencer, Instrument, Goal .
• Raju(AGENT) hit us (PATIENT) with a ball
  (INSTRUMENT).
• Complicated by the notions of direct and indirect
  object, active and passive voice.

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Ambiguities (Cont.)

• Attachment ambiguities occur with phrases that
  could have been generated by two different nodes
  in the parse tree. E.g. saw the man in the house
  with a pole.
• Rare Usage and spurious usage A hectare is a
  hundred ares.

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Garden-Path Sentences

• Garden-Path sentences are sentences that lead you
  along a path that suddenly turns out not to work.
  
  E.g. The horse raced past the barn fell.

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Morphology

• Study of the way words are built up from smaller
  meaning bearing units.
• Plural forms, Inflections, Derivation,
  Compounding.
• (come, comes, coming)
• (break, broke, broken)
• (happy, happier, happiest)
• ??(See, Seed)??

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Semantics and Pragmatics

• Semantics concerns what words mean and how these
  meanings combine in sentences to form sentence
  meanings.
• Pragmatics concerns how sentences are used in
  different situations and how use affects the
  interpretation of the sentence.
• Issues of Naturalness, acceptability and
  correctness in a given situation, determining,
  what is the speaker actually saying etc.