{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

class09-im

# class09-im - Three classic approaches to IR 1 Recall...

This preview shows pages 1–8. Sign up to view the full content.

Three “classic” approaches to IR 1 Recall: Boolean Retrieval 1 if play contains word, 0 otherwise Brutus AND Caesar but NOT Calpurnia 2

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Recall: Vector Space Retrieval 3 Probabilistic IR Chapter 11 Traditional Probabilistic IR model Traditionally: neat ideas, but they’ve never won on performance. Chapter 12 Statistical Language Models Very hot right now 4
Why probabilities in IR? User Information Need Documents Document Representation Query Representation How to match? In traditional 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? Uncertain guess of whether document has relevant content Understanding of user need is uncertain 5 But first ... Probability review Independent events Let a, b be two events, with probability P ( a ) and P ( b ). The events a and b are independent if and only if: P ( a ! b ) = P ( a ) P ( b ) In general, a 1 , a 2 , ... , a n are independent if and only if: P ( a 1 ! a 2 ! ... ! a n ) = P ( a 1 ) P ( a 2 )... P ( a n ) 6

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Probability review Let a, b be two events, with probability P ( a ) and P ( b ). Conditional probability P ( a | b ) is the probability of a given b, also called the conditional probability of a given b . Conditional independence The events a 1 , ..., a n are conditionally independent if and only if: P ( a i | a j ) = P(a i ) for all i and j. 7 Example Independent a and b are the results of throwing two dice P ( a =5 | b =3) = P ( a =5) = 1 /6 Not independent a and b are the results of throwing two dice t is the sum of the two dice t = a + b P ( t =8 | a =2) = 1 /6 P ( t =8 | a =1) = 0 8
Example P ( a ) = x + y P ( b ) = w + x P(a | b) = x / ( w + x ) P ( a | b) P ( b ) = P ( a ! b ) = P ( b | a ) P ( a ) a b w z y x a b where a is the event not a 9 Bayes theorem Notation Let a, b be two events. P ( a | b ) is the probability of a given b Bayes Theorem P ( a | b ) = Derivation P ( a | b) P ( b ) = P ( a ! b ) = P ( b | a ) P ( a ) P ( b | a ) P ( a ) P ( b ) 10

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Bayes theorem Terminology used with Bayes Theorem P ( a | b ) = P ( a ) is called the prior probability of a P ( a | b ) is called the posterior probability of a given b P ( b | a ) P ( a ) P ( b ) 11 Example of Bayes theorem Example a Weight over 200 lb. b Height over 6 ft. Over 200 lb Over 6 ft w z y x P ( a | b ) = x / ( w + x ) = x / P ( b ) P ( b | a ) = x / ( x + y ) = x / P ( a ) x is P ( a ! b ) 12
IR based on Language Model (LM) query d1 d2 dn Information need document collection generation A common search heuristic is to use words that you expect to find in matching documents as your query The LM approach directly exploits that idea!

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### What students are saying

• As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

Kiran Temple University Fox School of Business ‘17, Course Hero Intern

• I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

Dana University of Pennsylvania ‘17, Course Hero Intern

• The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

Jill Tulane University ‘16, Course Hero Intern