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Lecture 2: Retrieval Models. Maya Ramanath. QQ1. Vector space model: 0 for non-presence of a term, 1 for presence: Query: q1 AND q2 AND q3 Compare the set of results returned by the vector space model and boolean model. Term weighting (1/3): tf. Query: Cat. D2. D3. D1. D1. D3.

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Presentation Transcript
slide2
QQ1
  • Vector space model: 0 for non-presence of a term, 1 for presence:
  • Query: q1 AND q2 AND q3

Compare the set of results returned by the vector space model and boolean model.

term weighting 1 3 tf
Term weighting (1/3): tf

Query: Cat

D2

D3

D1

D1

D3

Query: Lion

D2

D1

D3

Query: The Lion

D2

D3

D2

D1

The lion (Pantheraleo) is one of four big cats …

Highly distinctive, the male lion is easily recognised by its mane…

The cat (Feliscatus), also known as the domestic cat …

Cats are similar in anatomy to the other felids…

The New World monkeys are classified within the parvorderPlatyrrhini, whereas the Old World monkeys form part of the parvorderCatarrhini, which also includes the hominoids…

term weighting 2 3 tf idf
Term weighting (2/3): tf.idf

Query: Cat

D2

D3

D1

D1

D3

Query: Lion

D2

D1

D3

Query: The Lion

D2

D3

D2

D1

The lion (Pantheraleo) is one of four big cats …

Highly distinctive, the male lion is easily recognised by its mane…

The cat (Feliscatus), also known as the domestic cat …

Cats are similar in anatomy to the other felids…

The New World monkeys are classified within the parvorderPlatyrrhini, whereas the Old World monkeys form part of the parvorderCatarrhini, which also includes the hominoids…

term weighting 3 3 doc length
Term weighting (3/3): doc length
  • Shorter the text, more important the match
  • Longer the text, more likely you “accidentally” fine a match

“Let me tell you about all the animals in the whole world (including the cat) !”

“Let me tell you about the cat, a domestic animal”

binary independence model
Binary Independence Model
  • Rank documents in decreasing probability of relevance

Derivation is long, but not difficult!

slide8

Let

Let

if

we still need relevant irrelevant documents
We still need relevant/irrelevant documents
  • Sample of corpus, exhaustively judged
  • Relevance feedback
  • Pseudo-relevance feedback
  • 2-poisson model
intuition 1 2
Intuition (1/2)

Document Q

Document D

Can we figure out the source?

This is the observation

intuition 2 2
Intuition (2/2)

PD

  • PQ

Document D

Document Q

These are the observations

Can we estimate PD and PQ?

query as a sample
Query as a sample

Estimated using MLE

references
References
  • For term weighting and the long derivation
    • Introduction to Information Retrieval: Raghavan, Manning and Shuetze, Cambridge University Press, 2008. Also available from: http://nlp.stanford.edu/IR-book/html/htmledition/irbook.html
  • Language Models
    • Statistical Language Models for Information Retrieval: A Critical Review. ChengXiangZhai, Foundations and Trends in IR 2(3), 2008
    • Also available in the IR book above