Prepositional Phrase Attachment Using Backoff

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Prepositional Phrase Attachment Using Back-off

• Konstantin Zak
• EECS 595
• 12/8/2005

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Problem

• See the cat with a telescope.
• Prepositional phrase with a telescope can belong
  to
• the noun cat
• or to verb see

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Related Work

• Eric Brill and Phil Resnik
• A Rule Based Approach to Prepositional Phrase
  Attachment Disambiguation Solution in 1994.
• corpus based approach

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Solution

• Prepositional Phrase Attachment through a
  Backed-Off Model
• Michael Collins and James Brooks
• 1995
• Statistical Approach
• Look at head words
• v n1 p n2

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Training and Test Data

• Quintuples
• 1 buy car with horn
• a 1 v buy n1 car p with n2 horn
• 1 designates noun attachment, 0 designates verb
  attachment
• 20801 training quintuples
• 3097 test quintuples

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Bounds on Performance

• Reasonable Lower Bound 72.2
• Reasonable Upper Bound 88.2
• Cant expect machine to do better then human

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Algorithm
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1 buy car with horn

• Quadruple
• 1buycarwithhorn
• Triples
• 1buycarwith
• 1buywithhorn
• 1carwithhorn
• Doubles
• 1buywith
• 1carwith
• 1withhorn
• Singles
• 1with

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Low Count Events

• Important to use stage with most elements
• If there are 100 instances of buywith, and only
  one instance of buycarwith, the data from the
  triple must be used
• Ignoring variables that have count of less then 5
  decreases accuracy to 81.6

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Results
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Added Feature

• Collins and Brooks recommend not ignoring any
  events
• Now ignores single instances of quadruples
• if f(1,v1,n1,p,n2) lt 2
• f(1,v1,n1,p,n2) 0
• if f(0,v1,n1,p,n2) lt 2
• f(0,v1,n1,p,n2) 0

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Comparison
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Improvements

• Increase training size
• If no words in test quintuple were seen in
  training data, default to noun attachment
• 60 accuracy

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Resources

• Michael Collins and James Brooks. 1995.
  Prepositional Phrase Attachment through a
  Backed-off Model. Proceedings of the Third
  Workshop on Very Large Corpora.
• http//www.comp.leeds.ac.uk/Perl/