Week1-5Why is NLP hard?

Examples

Time flies like an arrow.

Interpretations

More Examples

Syntax vs. semantics

*Little a has Mary lamb

• Syntactically wrong, markd as *

?Colorless green idea sleeps furiously

• Syntactically right, but semantically wrong, marked as ?
• idea doesn’t sleep
• sleep furiously?
• colorless green?
• green idea?

Ambiguous words

• ball, board, plant
  
  • meaning
• fly, rent, tape
  
  • part of speech
• address, resent, entrance, number, unionized
  
  • different meanings

Ambiguity

• Not in computer language(by design)!
• Lojban
• Noun-noun phrase: (XY)Z vs. X(YZ)
  
  • science fiction writer
  • state chess tournament

Types of ambiguity

• Morphological
• Phonetic
• Part of speech
• Syntactic
• PP attachment
• Sense
• Modality
• Subjectivity
• CC(coordinating conjunction) attachment
• Negation
• Referential
• Reflexive
• Ellipsis and parallelism
• Metonymy

Other sources of difficulties

• Non-standard, slang, novel words and usages
• Inconsistensies
• Typos and grammatical errors
• Parsing problems
• Complex sentences
• Counterfactural sentences
• Humor and sacarsm
• Implicature/inference/world knowledge
• Semantics and pragmatics
• Language is even hard for humans

Synonyms and paraphrases