Wednesday, 20 May 2020

Natural language processing question bank 05

List down the features of Probabilistic Context Free Grammars (PCFGs)


Question:

What are the features of Probabilistic Context Free Grammars?


Answer:


  • Partial solution for grammar ambiguity. A PCFG gives some idea of different reasonable parses.
  • PCFG is not lexicalized since its probability estimates are based purely on structural factors.
  • Robustness – the problem of grammatical mistakes, and errors in the real texts can be avoided by assigning low probability for non-reasonable (not probable) sentences. That is, PCFG admits everything with low probability.
  • Gives a probabilistic language model for English.
  • Predictive power of PCFG is comparatively better than that of HMM.
  • An n-gram language model considers some local lexical context while measuring the probability of a sentence, whereas a PCFG does not consider the lexical information. Hence, PCFG is worse model than any n-gram model for English.
  • In PCFG, the probability of smaller trees are greater than the larger trees.



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