Applying HPSG with Relaxed Unification to Question Answering

1
Applying HPSG with Relaxed Unification to
Question Answering
Faculty ofComputerScience
Tony Abou-Assaleh, Nick Cercone, and Vlado
Keeljtaa,nick,vlado_at_cs.dal.ca

• INTRODUCTION
• Motivation
• Head-driven Phrase Structure Grammar (HPSG) can
  represent both syntax and semantics
• Question Answering (QA) plays an important role
  in Natural Language Processing (NLP)
• Relaxed Unification enables HPSG to process data
  that is inconsistent, incomplete, and uncertain.
• Objectives
• Identify limitations of current QA systems
• Develop a framework to overcome these limitations
• Build a QA advanced prototype
• Objectively evaluate the prototype against other
  approaches

• EVALUATION
• Environment
• Text REtrieval Conference (TREC) Question
  Answering Track.
• Open-domain, closed-class questions.
• Shallow semantic analysis is some times required.
• Restricted QA only factoid and definition
  questions.
• Approach
• Use an Information Retrieval (IR) search engine
  to locate relevant passages.
• Use HPSG to parse the question and generate a
  query.
• Use HPSG to parse the passage.
• Use relaxed unification to locate a query match
  in the passage.
• Compute the correctness of the result using a
  metric to rank the answers.

RELAXED UNIFICATION
HPSG AND QUESTION ANSWERING

• Classical Unification
• Two terms, t and u, are unifiableif and only if
  there exists a substitution ? such that t ? u
  ?.
• Relaxed Unification
• Relaxed Term consists of variables and sets of
  functions, where sets can be empty. Function
  arguments are relaxed terms.E.g. f ( a ,
  h ( b ) ) .
• two terms, t and u, are always unifiable with a
  unifying substitution ? such that t ? u ?.
• Application-specific correctness measure
  evaluates the result.

• Query
• How much could you rent a Volkswagen bug for in
  1966?
• Relevant Text
• you could rent a Volkswagen bug for 1 a
  day.
• Answer
• 1 a day.