1. Lecture 2: Retrieval Models Maya Ramanath

2. 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.

3. Term weighting (1/3): tf The lion (Panthera leo) is one of four big cats … Highly distinctive, the male lion is easily recognised by its mane… The lion (Panthera leo) is one of four big cats … Highly distinctive, the male lion is easily recognised by its mane… The cat (Felis catus), also known as the domestic cat … Cats are similar in anatomy to the other felids… The cat (Felis catus), also known as the domestic cat … Cats are similar in anatomy to the other felids… The New World monkeys are classified within the parvorder Platyrrhini, whereas the Old World monkeys form part of the parvorder Catarrhini, which also includes the hominoids… DOCCOUNT LionCatThe D1212 D2032 D3005 D1 D2 D3 Query: Cat Query: Lion Query: The Lion D2 D1 D3 D1 D2 D3 D1 D2 D3

4. Term weighting (2/3): tf.idf The lion (Panthera leo) is one of four big cats … Highly distinctive, the male lion is easily recognised by its mane… The lion (Panthera leo) is one of four big cats … Highly distinctive, the male lion is easily recognised by its mane… The cat (Felis catus), also known as the domestic cat … Cats are similar in anatomy to the other felids… The cat (Felis catus), also known as the domestic cat … Cats are similar in anatomy to the other felids… The New World monkeys are classified within the parvorder Platyrrhini, whereas the Old World monkeys form part of the parvorder Catarrhini, which also includes the hominoids… DOCCOUNT LionCatThe D1212 D2032 D3005 D1 D2 D3 Query: Cat Query: Lion Query: The Lion D2D2 D2D2 D1D1 D1D1 D3D3 D3D3 D1D1 D1D1 D2D2 D2D2 D3D3 D3D3 D1D1 D1D1 D2D2 D2D2 D3D3 D3D3 DOCWEIGHT LionCatThe D12/11/22/3 D203/22/3 D3005/3

5. 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 the cat, a domestic animal” “Let me tell you about all the animals in the whole world (including the cat) !”

6. PROBABILISTIC RANKING

7. Binary Independence Model Rank documents in decreasing probability of relevance Derivation is long, but not difficult!

8. Let if

10. We still need relevant/irrelevant documents Sample of corpus, exhaustively judged Relevance feedback Pseudo-relevance feedback 2-poisson model …

11. LANGUAGE MODELS

12. Intuition (1/2) Document D Document Q This is the observation Can we figure out the source?

13. Intuition (2/2) PDPD PDPD PQPQ PQPQ Document DDocument Q These are the observations Can we estimate P D and P Q ?

14. Query as a sample Estimated using MLE

15. 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.htmlhttp://nlp.stanford.edu/IR- book/html/htmledition/irbook.html Language Models – Statistical Language Models for Information Retrieval: A Critical Review. ChengXiang Zhai, Foundations and Trends in IR 2(3), 2008 – Also available in the IR book above

16. QUESTIONS ?