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# Language Models for Information Retrieval

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• --- Rah-guh-vun
• --- appears to be the most social and outgoing of our nerdy authors
• ---Q-d model: modeling the relevance of a document to a query
• Alright, so: what is a document model? And how does it generate the query?They use the concept of automata to help explain what is meant by a language or document model. For any given document you have an alphabet w.r.t. that document and a language produces by that alphabetProbability is distributed over terms ST the sum of all probabilities is equal to 1. straightforward.
• --- I didn’t quite understand where the 0.8 stop/continue probability came from---Left out because given a fixed STOP prob, it does not effect results when comparing models to leave it out.Now we will compare models
• Next we look at probability over sequences of terms.
• ---By using the chain rule, we can build probabilities over sequences of terms. ---Two specific models that use the chain rule are the unigram and bigram modelsDescribe images---The fundamental question in language modeling is which doc-model to use?
• ---now we introduce formally the model representing the initial concepts of LM for IR
• The most common way to achieve the goal of the query likelihood model is to use the multinomial unigram language modelThe query generation process is randomNext: estimating this 𝐏𝒒𝑴𝒅The most common way to achieve the goal of the query likelihood model is to use the multinomial unigram language modelThe query generation process is randomNext: estimating this 𝐏(𝒒│𝑴_𝒅 )
• Basically we are counting how often each word occurs and dividing by the total # of words in the documentNotice the ^, that indicates that this probability is an estimateTherein lies the issue with language modelsWhich leads to the re-occuring issue of “zero probabilities”Which then leads to the much used approach of “smoothing”, which we will see a lot of in the next two presentations in detail.
• the initial idea behind smoothing was to allow for non-occuring terms to be in a query generated by the document model GIVE example, say you have a document about tigers that doesn’t contain the word cat but a user queries “big striped cats”One of the important points in this section is that smoothing is essential for the overall good properties of LMs
• ---But, as Dr. Lease has mentioned… its easy to get good results when you are comparing to the standard tf-idf---NEXT: comparison of language models to other IR approaches
• But they mention that LM can be thought to indirectly include relevance modeling by viewing documents and info needs as the same type of object and analyzing it with NLP BIM = binary independence model
• -Both use tf-Both use df and cf to produce prob-Both treat terms independently ------NEXT: document model
• Downsides: both downside stem from there being less text to estimate withNEXT: all three approaches
• --- so far we’ve addressed query likelihood and document likelihood, now they focus on comparing these modelsNext: model comparison
• Q -- What will we use to compare models? One example would be the notorious KL-divergence.Comment -- Some prior results show that comparing models outperforms both query and document likelihood modelsComment -- Not bad for ad hoc queries, but bad for topic trackingNEXT: translation model
• -- Synonymy: uses similar, but not the same words to say the same thing---I believe synonymy is still a pretty big issue
• -- more computationally intensive than basic LM approaches-- all of these extended language models have been shown to improve basic LM approaches
• ### Language Models for Information Retrieval

1. 1. Introduction to Information Retrieval:Language models for information retrievalby C.D. Manning, P. Raghavan, and H. Schutze.<br />Presentation by Dustin Smith<br />The University of Texas at Austin<br />School of Information<br />dustin.smith@utexas.edu<br />10/3/2011<br />1<br />INF384H / CS395T: Concepts of Information Retrieval<br />
2. 2. Christopher Manning – background<br />BA Australian National University 1989 (majors in mathematics, computer science and linguistics)<br />PhD Stanford Linguistics 1995<br />Asst Professor Carnegie Mellon University Computational Linguistics Program 1994-96<br />Lecturer University of Sydney Dept of Linguistics 1996-99<br />Asst Professor Stanford University Depts of Computer Science and Linguistics 1999-2006<br />Current: Assoc Professor Stanford University Depts of Linguistics and Computer Science<br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />2<br />
3. 3. Prabhakar Raghavan– background<br />Undergraduate degree in electrical engineering from ITT, Madras<br />PhD in computer science from UC Berkeley<br />Current: Working at Yahoo! Labs and is a Consulting Professor of Computer Science at Stanford University<br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />3<br />
4. 4. Hinrich Schütze– background<br />Technical University of Braunschweig<br />Vordiplom Mathematik<br />Vordiplom Informatik<br />University of Stuttgart, Diplom Informatik (MSCS)<br />Stanford University, Ph.D., Computational Linguistics<br />Current: Chair of Theoretical Computational Linguistics, Institute for Natural Language Processing at the University of Stuttgart<br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />4<br />
5. 5. Chapter/Presentation Outline<br />Introduction to the concept of Language Models<br />Finite automata and language models<br />Types of language models<br />Multinomial distributions over words<br />Description of the Query Likelihood Model<br />Using query likelihood language models in IR<br />Estimating the query generation probability<br />Ponte and Croft’s experiments<br />Comparison of the language modeling approach to IR against other approaches to IR<br />Description of various extension to the language modeling approach<br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />5<br />
6. 6. Language Models<br />Based on concept that a document is a good match for a query if the document model is likely to generate the query.<br />An alternative to the straightforward query-document probability model. (traditional approach)<br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />6<br />
7. 7. Finite automata and language models (238)<br /><ul><li>In figure 12.1 the alphabet is {“I”, “wish”} and the language produced by the model is {“I wish”, “I wish I wish”, “I wish I wish I wish I wish”, etc.}
8. 8. The process is analogous for a document model
9. 9. Figure 12.2 represents a single node with a single distribution over terms s.t.𝑡∈𝑉𝑃(𝑡)=1. </li></ul> <br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />7<br />Language Models<br />
10. 10. Calculating phrase probability with stop/continue probability included (238)<br /><ul><li>The probability calculations are very small.
11. 11. This calculation is shown with stop probabilities, but in practice these are left out. </li></ul>10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />8<br />Language Models<br />
12. 12. Comparison of document models (239-240)<br /><ul><li>In theory these models represent different documents, different alphabets, and different languages.
13. 13. Given a query s = “frog said that toad likes that dog”,our two model probabilities are calculated by simply multiplying term distributions.
14. 14. It’s evident why P(s|𝑀1) scores higher than P(s|𝑀2). More query terms were present in P(s|𝑀1) and so the probability is greater.</li></ul> <br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />9<br />Language Models<br />
15. 15. Types of language models(240)<br /><ul><li>Unigram Language Model
16. 16. Bigram Language Model
17. 17. Section Conclusion
18. 18. Which𝑀𝑑 to use?</li></ul> <br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />10<br />Chain rule<br />Language Models<br />
19. 19. Using query likelihood language models in IR (242-243)<br />Using Bayes rule:<br />P(d|q)=P(q|d)P(d)/P(q)<br />With P(d) and P(q) uniform across documents,<br />=> P(d|q) = P(q|d)<br />In the query likelihood model we construct a language model 𝑀𝑑 from each document<br />Goal: to rank documents by P(d|q), where the probability of a document is interpreted as the likelihood that it is relevant to the query<br /> <br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />11<br />The Query Likelihood Model<br />
20. 20. Using query likelihood language models in IR (242-243)<br />Multinomial unigram language model<br />Pq𝑀𝑑=𝐾𝑞𝑡∈𝑉𝑃(𝑡|𝑀𝑑)𝑡𝑓𝑡,𝑑<br />𝐾𝑞 is dropped as it is constant across all queries<br /> <br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />12<br />Query generation process:1. Infer a LM for each document <br />2. Estimate Pq𝑀𝑑, the probability of generating the query according to each one of these document models<br />3. Rank the documents according to these probabilities<br /> <br />The Query Likelihood Model<br />
21. 21. Estimating the query generation probability (244)<br /><ul><li>Query generation probability = Pq𝑀𝑑</li></ul> <br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />13<br /><ul><li>M𝑑 is the language model of document d
22. 22. tf𝑡.𝑑is the raw term frequency of term t in document d
23. 23. L𝑑is the number of tokens in document d</li></ul> <br />The Query Likelihood Model<br />
24. 24. Smoothing Methods (245-246)<br /><ul><li>Linear Interpolation
25. 25. Bayesian Smoothing
26. 26. Note: MLE =
27. 27. maximum likelihood estimate</li></ul>Conceptually the same:<br />The probability estimate for a word present in the document combines a discounted (MLE) and a fraction of the estimate of its prevalence in the whole collection.<br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />14<br />
28. 28. Ponte and Croft’s Experiments (246)<br /><ul><li>1998 experiments
29. 29. First experiments on the language modeling approach to IR
30. 30. Performed on TREC topics 202-250 over TREC disks 2 and 3.</li></ul>LM much better than tf-idf (specifically at higher recalls) <br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />15<br />The Query Likelihood Model<br />
31. 31. LM vs. BIM vs. XML retrieval (249)<br />Language models and the most successful XML retrieval models approach relevance modeling in a roundabout way as apposed to the BIM model that evaluates relevance directly.<br />LM initially appears to not include relevance modeling <br />The most successful XML retrieval models assume that queries and documents are objects of the same type<br />BIM models have relevance as the central variable that is evaluated<br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />16<br />Language Modeling Versus Other Approaches in IR<br />
32. 32. LM vs. traditional tf-idf(249)<br />The LM has significant relations to tf-idf models <br />They differ on a more conceptual level<br />Both directly use term frequency<br />Both have a method of mixing document frequency and collection frequency to produce probabilities<br />Both treat terms independently<br />LM intuitions are more probabilistic than geometric<br />LM mathematical models are more principled rather than heuristic<br />LM differs in its use of tf and df<br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />17<br />
33. 33. Document Likelihood Model (250)<br />Downsides:<br /><ul><li>Takes much more smoothing
34. 34. Results in worse estimates</li></ul>Features:<br /><ul><li>Uses a query to generate a document with a query language model (𝑀𝑞)
35. 35. Easier to incorporate relevance feedback by expanding the query with terms from relevant documents</li></ul> <br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />18<br />Extended Language Modeling Approaches<br />
36. 36. Three language model approaches (250)<br /><ul><li>Query likelihood
37. 37. Using a document model to produce a relevant query
38. 38. Document likelihood
39. 39. Using a query model to produce a relevant document
40. 40. Model comparison
41. 41. Comparing these models </li></ul>10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />19<br />Extended Language Modeling Approaches<br />
42. 42. Kullback-Leibler (KL) divergence (251)<br /><ul><li>KL divergence is an asymmetric divergence measure originating in information theory, which measures how bad the probability distribution 𝑀𝑞 is at modeling 𝑀𝑑 (pg. 251)
43. 43. Outperforms query and document likelihood models
44. 44. But, scores are not comparable across queries</li></ul> <br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />20<br />Extended Language Modeling Approaches<br />
45. 45. Translation Model – Features (251)<br />Answer to synonymy in basic LM models<br />Lets you generate query words that are not in a document by translating to alternate terms with similar meaning<br />Provides a basis for executing cross-language IR<br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />21<br />Extended Language Modeling Approaches<br />
46. 46. Translation Model – Issues (251)<br />Computationally intensive <br />Need to build the model using outside resources<br />Thesaurus <br />Bilingual dictionary<br />Statistical machine translation system’s translation dictionary<br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />22<br />Extended Language Modeling Approaches<br />
47. 47. Thanks for not throwing vegetables!<br />Questions?<br />10/3/2011<br />INF384H / CS395T: Concepts of Information Retrieval<br />23<br />