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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

### Transcript

• 1. Introduction to Information Retrieval:Language models for information retrievalby C.D. Manning, P. Raghavan, and H. Schutze.
Presentation by Dustin Smith
The University of Texas at Austin
School of Information
dustin.smith@utexas.edu
10/3/2011
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INF384H / CS395T: Concepts of Information Retrieval
• 2. Christopher Manning – background
BA Australian National University 1989 (majors in mathematics, computer science and linguistics)
PhD Stanford Linguistics 1995
Asst Professor Carnegie Mellon University Computational Linguistics Program 1994-96
Lecturer University of Sydney Dept of Linguistics 1996-99
Asst Professor Stanford University Depts of Computer Science and Linguistics 1999-2006
Current: Assoc Professor Stanford University Depts of Linguistics and Computer Science
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• 3. Prabhakar Raghavan– background
PhD in computer science from UC Berkeley
Current: Working at Yahoo! Labs and is a Consulting Professor of Computer Science at Stanford University
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• 4. Hinrich Schütze– background
Technical University of Braunschweig
Vordiplom Mathematik
Vordiplom Informatik
University of Stuttgart, Diplom Informatik (MSCS)
Stanford University, Ph.D., Computational Linguistics
Current: Chair of Theoretical Computational Linguistics, Institute for Natural Language Processing at the University of Stuttgart
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• 5. Chapter/Presentation Outline
Introduction to the concept of Language Models
Finite automata and language models
Types of language models
Multinomial distributions over words
Description of the Query Likelihood Model
Using query likelihood language models in IR
Estimating the query generation probability
Ponte and Croft’s experiments
Comparison of the language modeling approach to IR against other approaches to IR
Description of various extension to the language modeling approach
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• 6. Language Models
Based on concept that a document is a good match for a query if the document model is likely to generate the query.
An alternative to the straightforward query-document probability model. (traditional approach)
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• 7. Finite automata and language models (238)
• 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. The process is analogous for a document model
• 9. Figure 12.2 represents a single node with a single distribution over terms s.t.𝑡∈𝑉𝑃(𝑡)=1.

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Language Models
• 10. Calculating phrase probability with stop/continue probability included (238)
• The probability calculations are very small.
• 11. This calculation is shown with stop probabilities, but in practice these are left out.
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Language Models
• 12. Comparison of document models (239-240)
• In theory these models represent different documents, different alphabets, and different languages.
• 13. Given a query s = “frog said that toad likes that dog”,our two model probabilities are calculated by simply multiplying term distributions.
• 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.

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Language Models
• 15. Types of language models(240)
• Unigram Language Model
• 16. Bigram Language Model
• 17. Section Conclusion
• 18. Which𝑀𝑑 to use?

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Chain rule
Language Models
• 19. Using query likelihood language models in IR (242-243)
Using Bayes rule:
P(d|q)=P(q|d)P(d)/P(q)
With P(d) and P(q) uniform across documents,
=> P(d|q) = P(q|d)
In the query likelihood model we construct a language model 𝑀𝑑 from each document
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

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The Query Likelihood Model
• 20. Using query likelihood language models in IR (242-243)
Multinomial unigram language model
Pq𝑀𝑑=𝐾𝑞𝑡∈𝑉𝑃(𝑡|𝑀𝑑)𝑡𝑓𝑡,𝑑
𝐾𝑞 is dropped as it is constant across all queries

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Query generation process:1. Infer a LM for each document
2. Estimate Pq𝑀𝑑, the probability of generating the query according to each one of these document models
3. Rank the documents according to these probabilities

The Query Likelihood Model
• 21. Estimating the query generation probability (244)
• Query generation probability = Pq𝑀𝑑

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• M𝑑 is the language model of document d
• 22. tf𝑡.𝑑is the raw term frequency of term t in document d
• 23. L𝑑is the number of tokens in document d

The Query Likelihood Model
• 24. Smoothing Methods (245-246)
• Linear Interpolation
• 25. Bayesian Smoothing
• 26. Note: MLE =
• 27. maximum likelihood estimate
Conceptually the same:
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.
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• 28. Ponte and Croft’s Experiments (246)
• 1998 experiments
• 29. First experiments on the language modeling approach to IR
• 30. Performed on TREC topics 202-250 over TREC disks 2 and 3.
LM much better than tf-idf (specifically at higher recalls)
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The Query Likelihood Model
• 31. LM vs. BIM vs. XML retrieval (249)
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.
LM initially appears to not include relevance modeling
The most successful XML retrieval models assume that queries and documents are objects of the same type
BIM models have relevance as the central variable that is evaluated
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Language Modeling Versus Other Approaches in IR
• 32. LM vs. traditional tf-idf(249)
The LM has significant relations to tf-idf models
They differ on a more conceptual level
Both directly use term frequency
Both have a method of mixing document frequency and collection frequency to produce probabilities
Both treat terms independently
LM intuitions are more probabilistic than geometric
LM mathematical models are more principled rather than heuristic
LM differs in its use of tf and df
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• 33. Document Likelihood Model (250)
Downsides:
• Takes much more smoothing
• 34. Results in worse estimates
Features:
• Uses a query to generate a document with a query language model (𝑀𝑞)
• 35. Easier to incorporate relevance feedback by expanding the query with terms from relevant documents

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Extended Language Modeling Approaches
• 36. Three language model approaches (250)
• Query likelihood
• 37. Using a document model to produce a relevant query
• 38. Document likelihood
• 39. Using a query model to produce a relevant document
• 40. Model comparison
• 41. Comparing these models
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Extended Language Modeling Approaches
• 42. Kullback-Leibler (KL) divergence (251)
• 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. Outperforms query and document likelihood models
• 44. But, scores are not comparable across queries

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Extended Language Modeling Approaches
• 45. Translation Model – Features (251)
Answer to synonymy in basic LM models
Lets you generate query words that are not in a document by translating to alternate terms with similar meaning
Provides a basis for executing cross-language IR
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Extended Language Modeling Approaches
• 46. Translation Model – Issues (251)
Computationally intensive
Need to build the model using outside resources
Thesaurus
Bilingual dictionary
Statistical machine translation system’s translation dictionary
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Extended Language Modeling Approaches
• 47. Thanks for not throwing vegetables!
Questions?
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