Information Retrieval and Organisation

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Information Retrieval and Organisation

• Chapter 11
• Probabilistic Information Retrieval

Dell Zhang Birkbeck, University of London
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Why Probabilities in IR?
Query Representation
User Information Need
Understanding of user need is uncertain
How to match?
Uncertain guess of whether document has relevant
content
Document Representation
Documents
In IR systems, matching between each document and
query is attempted in a semantically imprecise
space of index terms. Probabilities provide a
principled foundation for uncertain
reasoning. Can we use probabilities to quantify
our uncertainties?
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Why Probabilities in IR?

• Problems with vector space model
• Ad-hoc term weighting schemes
• Ad-hoc basis vectors
• Ad-hoc similarity measurement
• We need something more principled!

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Probability Ranking Principle

• The document ranking method is the core of an IR
  system
• We have a collection of documents. The user
  issues a query. A list of documents needs to be
  returned.
• In what order do we present documents to the
  user? We want the best document to be first,
  second best second, etc.

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Probability Ranking Principle

• If a reference retrieval system's response to
  each request is a ranking of the documents in the
  collection in order of decreasing probability of
  relevance to the user who submitted the request,
  where the probabilities are estimated as
  accurately as possible on the basis of whatever
  data have been made available to the system for
  this purpose, the overall effectiveness of the
  system to its user will be the best that is
  obtainable on the basis of those data.

van Rijsbergen (1979113-114)
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Probability Ranking Principle

• Theorem. The PRP is optimal, in the sense that it
  minimizes the expected loss (also known as the
  Bayes risk) under 1/0 loss.
• Provable if all probabilities are known correctly.

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Binary Independence Model

• BIM is the model that has traditionally been used
  in conjunction with PRP
• Binary Boolean documents and queries are
  represented as binary incidence vectors of terms.
• Independence terms occur in documents and
  queries independently.

BIM Bernoulli Naive Bayes model
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Binary Independence Model

• Use Bayes Rule

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Binary Independence Model

• Rank documents by their odds

constant
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Binary Independence Model

• Make the Naïve Bayes conditional independence
  assumption

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Binary Independence Model

• Let

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Binary Independence Model

• Assume that

constant
constant
useful
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Binary Independence Model

• Taking the logarithm function which is monotonic,
  we get the retrieval status value

log odds ratio
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Binary Independence Model

• Assume that relevant documents are a very small
  percentage

IDF
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Okapi BM25

• Assume that
• Factor in the term frequencies (tf) and document
  length (Ld and Lave)

Ideally the parameters k1, b should be tuned on a
validation set.
Practically good vales 1.2 k1 2 b 0.75
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Appraisal

• Getting reasonable approximations of
  probabilities is possible, but requires
  restrictive assumptions. In the BIM, these are
• a Boolean representation of documents/queries/rele
  vance
• term independence
• terms not in the query dont affect the outcome
• document relevance values are independent
• Problem either require partial relevance
  information or only can derive somewhat inferior
  term weights.

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Appraisal

• Probabilistic methods are one of the oldest but
  also one of the currently hottest topics in IR.
• Traditionally neat ideas, but theyve never won
  on performance.
• It may be different now. For example, the Okapi
  BM25 term weighting formulas have been very
  successful, especially in TREC evaluations.

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Well-Known UK Researchers
Stephen Robertson
Keith van Rijsbergen
Karen Sparck Jones