Presented by: Ashraf Khalil

1. Indexing by Latent Semantic AnalysisScot Deerwester, Susan Dumais,George Furnas,Thomas Landauer, and Richard Harshman Presented by: Ashraf Khalil

2. Outline • The Problem • Some History • LSA • A Small Example • Efficiency • Other applications • Summary

3. The Problem • Given a collection of documents: retrieve documents that are relevant to a given query • Match terms in documents to terms in query

4. The Problem • The vector space method • term (rows) by document (columns) matrix, based on occurrence • translate into vectors in a vector space • one vector for each document • cosine to measure distance between vectors (documents) • small angle = large cosine = similar • large angle = small cosine = dissimilar

5. The Problem • Two problems that arose using the vector space model: • synonymy: many ways to refer to the same object, e.g. car and automobile • leads to poor recall • polysemy: most words have more than one distinct meaning, e.g. Jaguar • leads to poor precision

6. The Goal • Latent Semantic Indexing was proposed to address these two problems with the vector space model

7. Some History • Latent Semantic Indexing was developed at Bellcore (now Telcordia) in the late 1980s (1988). It was patented in 1989. • http://lsi.argreenhouse.com/lsi/LSI.html • The first papers about LSI: • Dumais, S. T., Furnas, G. W., Landauer, T. K. and Deerwester, S. (1988), "Using latent semantic analysis to improve information retrieval." In Proceedings of CHI'88: Conference on Human Factors in Computing, New York: ACM, 281-285. • Deerwester, S., Dumais, S. T., Landauer, T. K., Furnas, G. W. and Harshman, R.A. (1990) "Indexing by latent semantic analysis." Journal of the Society for Information Science, 41(6), 391-407.

8. LSA: The idea • Idea (Deerwester et al): • “We would like a representation in which a set of terms, which by itself is incomplete and unreliable evidence of the relevance of a given document, is replaced by some other set of entities which are more reliable indicants. We take advantage of the implicit higher-order (or latent) structure in the association of terms and documents to reveal such relationships.” • The assumption is that co-occurrence says something about semantics: words about the same things are likely to occur in the same contexts • If we have many words and contexts, small differences in co-occurrence probabilities can be compiled together to give information about semantics.

9. LSA: Overview • Build a matrix with rows representing words and columns representing context (a document or word string) • Apply SVD • unique mathematical decomposition of a matrix into the product of three matrices: • two with orthonormal columns-- (orthonormal)? • one with singular values on the diagonal • tool for dimension reduction • similarity measure based on co-occurrence • finds optimal projection into low-dimensional space

10. LSA Methods • Start with a Term-by-Document matrix • Optionally weight cells • Apply Singular Value Decomposition: • t = # of terms • d = # of documents • n = min(t, d) • Approximate using k (semantic) dimensions:

11. LSA: • SVD • can be viewed as a method for rotating the axes in n-dimensional space, so that the first axis runs along the direction of the largest variation among the documents • the second dimension runs along the direction with the second largest variation • and so on • generalized least-squares method

12. LSA • Rank-reduced Singular Value Decomposition (SVD) performed on matrix • all but the k highest singular values are set to 0 • produces k-dimensional approximation of the original matrix • this is the “semantic space” • Compute similarities between entities in semantic space (usually with cosine)

13. A Small Example Technical Memo Titles c1: Human machine interface for ABC computer applications c2: A survey of user opinion of computersystemresponsetime c3: The EPSuserinterface management system c4: System and humansystem engineering testing of EPS c5: Relation of user perceived responsetime to error measurement m1: The generation of random, binary, ordered trees m2: The intersection graph of paths in trees m3: Graphminors IV: Widths of trees and well-quasi-ordering m4: Graphminors: A survey

14. A Small Example – 2 r (human.user) = -.38 r (human.minors) = -.29

15. A Small Example – 3 • T =

16. A Small Example – 4 • S =

17. A Small Example – 5 • D =

18. A Small Example – 7

19. r (human.user) = .94 r (human.minors) = -.83 A Small Example – 7

20. A Small Example – 2 again r (human.user) = -.38 r (human.minors) = -.29

21. Correlation: Raw data 0.92 -0.72 1.00

22. Some Issues with LSI • SVD Algorithm complexity O(n^2k^3) • n = number of terms + documents • k = number of dimensions in semantic space (typically small ~50 to 350) • Although lot of empirical evidence no concrete proof of why LSI works

23. performance k Semantic Dimension • Finding optimal dimension for semantic space • precision-recall improve as dimension is increased until hits optimal, then slowly decreases until it hits standard vector model • in many tasks 150-350 works well, still room for research • choosing k is difficult • overfitting (superfluous dimensions) vs. underfitting (not enough dimensions)

24. Other Applications • Has proved to be a valuable tool in many areas as well as IR • summarization • cross-language IR • topics segmentation • text classification • question answering • LSA can pass the TOEFL

25. LSA can Pass the TOEFL • Task: • Multiple-choice test for synonym • Given one word, find best match out of 4 alternatives • Training: • Corpus of 30,473 articles from Grolier’s Academic • Used first ~150 words from each article => 60,768 unique • words that occur at least twice • 300 singular vectors • Result • LSI gets 52.5% correct (corrected for guessing) • Non-LSI similarity gets 15.8% (other paper 29.5%) correct • Average (foreign) human test taker gets 52.7% • Landauer, T. K. and Dumais, S. T. (1997) A solution to Plato's problem: the Latent Semantic Analysis theory of acquisition, induction and representation of knowledge. Psychological Review, 104(2) 211-240.

26. LSA can mark essays • LSA judgments of the quality of sentences correlate at r = 0.81 with expert ratings • LSA can judge how good an essay (on a well-defined set topic) is by computing the average distance between the essay to be marked and a set of model essays • The correlation are equal to between-human correlations • “If you wrote a good essay and scrambled the words you would get a good grade," Landauer said. "But try to get the good words without writing a good essay!”

27. Good References • The group at the University of Colorado at Boulder has a web site where you can try out LSA and download papers • http://lsa.colorado.edu/ • Papers are also available at: • http://lsi.research.telcordia.com/lsi/LSI.html