Context-Free Grammars and Constituency Parsing

19.5 Ambiguity

1. What is structural ambiguity?
   When a grammar can assign more than one parse to a sentence.
   • Jurafsky & Martin, Speech and Language Processing, 3rd ed. draft, ch. 19, §19.5, p. 433
2. Why is “I shot an elephant in my pajamas” structurally ambiguous?
   PP-attachment: in my pajamas can attach inside the NP headed by elephant (the elephant wears the pajamas) or to the VP headed by shot (the shooter wears them).

   NP attachment	shot [NP an elephant in my pajamas]
   VP attachment	[VP shot an elephant] in my pajamas

   • Jurafsky & Martin, Speech and Language Processing, 3rd ed. draft, ch. 19, §19.5, p. 434
3. What is attachment ambiguity? Give a non-PP example.
   A constituent can attach to the tree at more than one place. E.g. in “We saw the Eiffel Tower flying to Paris”, the gerundive VP flying to Paris can belong to a clause whose subject is the Eiffel Tower, or modify saw.
   • Jurafsky & Martin, Speech and Language Processing, 3rd ed. draft, ch. 19, §19.5, p. 434
4. What is coordination ambiguity?
   Ambiguity in what a conjunction conjoins: old men and women = [old [men and women]] or [[old men] and women].
   • Jurafsky & Martin, Speech and Language Processing, 3rd ed. draft, ch. 19, §19.5, p. 434
5. Why is ambiguity such a serious problem for parsers?
   Real sentences contain many grammatically valid but semantically unreasonable parses, combining attachment and coordination ambiguities; a parser must handle them efficiently (CKY) and choose the right one (syntactic disambiguation).
   • Jurafsky & Martin, Speech and Language Processing, 3rd ed. draft, ch. 19, §19.5, p. 435