Chapter 5: Query Operations

1. Chapter 5: Query Operations Baeza-Yates, 1999 Modern Information Retrieval

2. Query Modification • Improving initial query formulation • Relevance feedback • approaches based on feedback information from users • Local analysis • approaches based on information derived from the set of documents initially retrieved (called the local set of documents) • Global analysis • approaches based on global information derived from the document collection

3. Relevance Feedback • Relevance feedback process • it shields the user from the details of the query reformulation process • it breaks down the whole searching task into a sequence of small steps which are easier to grasp • it provides a controlled process designed to emphasize some terms and de-emphasize others • Two basic techniques • Query expansion • addition of new terms from relevant documents • Term reweighting • modification of term weights based on the user relevance judgement

4. Vector Space Model • Definitionwi,j: the ith term in the vector for document djwi,k: the ith term in the vector for query qkt: the number of unique terms in the data set

5. Query Expansion and and Term Reweighting for the Vector Model • Ideal situation • CR: set of relevant documents among all documents in the collection • Rocchio (1965, 1971) • R: set of relevant documents, as identified by the user among the retrieved documents • S: set of non-relevant documents among the retrieved documents

6. Rocchio’s Algorithm • Ide_Regular (1971) • Ide_Dec_Hi • Parameters • a = b = g =1 • b > g = 0

7. Probabilistic Model • Definition • pi: the probability of observing term ti in the set of relevant documents • qi: the probability of observing term ti in the set of nonrelevant documents • Initial search assumption • pi is constant for all terms ti (typically 0.5) • qi can be approximated by the distribution of ti in the whole collection

8. - + ni-ri ni ri N-ni R-ri N N-R R Term Reweighting for the Probabilistic Model • Robertson and Sparck Jones (1976) • With relevance feedback from user N: the number of documents in the collection R: the number of relevant documents for query q ni: the number of documents having term ti ri: the number of relevant documents having term ti Document Relevance Document Indexing + - N-ni-R+ri

9. Term Reweighting for the Probabilistic Model (cont.) • Initial search assumption • pi is constant for all terms ti (typically 0.5) • qi can be approximated by the distribution of ti in the whole collection • With relevance feedback from users • pi and qi can be approximated by • hence the term weight is updated by

10. Term Reweighting for the Probabilistic Model (Cont.) • However, the last formula poses problems for certain small values of R andri(R=1, ri=0) • Instead of 0.5, alternative adjustments have been propsed

11. Term Reweighting for the Probabilistic Model (Cont.) • Characteristics • Advantage • the term reweighting is optimal under the asumptions of • term independence • binary document indexing (wi,q {0,1} and wi,j {0,1}) • Disadvantage • no query expansion is used • weights of terms in the previous query formulations are also disregarded • document term weights are not taken into account during the feedback loop

12. Evaluation of relevance feedback • Standard evaluation method is not suitable • (i.e., recall-precision) because the relevant documents used to reweight the query terms are moved to higher ranks. • The residual collection method • the set of all documents minus the set of feedback documents provided by the user • because highly ranked documents are removed from the collection, the recall-precision figures for tend to be lower than the figures for the original query • as a basic rule of thumb, any experimentation involving relevance feedback strategies should always evaluate recall-precision figures relative to the residual collection

13. Automatic Local Analysis • Definition • local document set Dl : the set of documents retrieved by a query • local vocabulary Vl : the set of all distinct words in Dl • stemed vocabulary Sl: the set of all distinct stems derived from Vl • Building local clusters • association clusters • metric clusters • scalar clusters

14. Association Clusters • Idea • co-occurrence of stems (or terms) inside documents • fu,j: the frequency of a stem ku in a document dj • local association cluster for a stem ku • the set of k largest values c(ku, kv) • given a query q, find clusters for the |q| query terms • normalized form

15. Metric Clusters • Idea • consider the distance between two terms in the same cluster • Definition • V(ku): the set of keywords which have the same stem form as ku • distance r(ki, kj)=the number of words between term ku and kv • normalized form

16. Scalar Clusters • Idea • two stems with similar neighborhoods have some synonymity relationships • Definition • cu,v=c(ku, kv) • vectors of correlation values for stem ku and kv • scalar association matrix • scalar clusters • the set of k largest values of scalar association

17. Automatic Global Analysis • A thesaurus-like structure • Short history • Until the beginning of the 1990s, global analysis was considered to be a technique which failed to yield consistent improvements in retrieval performance with general collections • This perception has changed with the appearance of modern procedures for global analysis

18. Query Expansion based on aSimilarity Thesaurus • Idea by Qiu and Frei [1993] • Similarity thesaurus is based on term to term relationships rather than on a matrix of co-occurrence • Terms for expansion are selected based on their similarity to the whole query rather than on their similarities to individual query terms • Definition • N: total number of documents in the collection • t: total number of terms in the collection • tfi,j: occurrence frequency of term ki in the document dj • tj: the number of distinct index terms in the document dj • itfj : the inverse term frequency for document dj

19. Similarity Thesaurus • Each term is associated with a vector • where wi,j is a weight associated to the index-document pair • The relationship between two terms ku and kv is • Note that this is a variation of the correlation measure used for computing scalar association matrices

20. Term weighting vs. Term concept space Doc dj Term ki Doc dj tfij tfij Term ki

21. Query Expansion Procedure with Similarity Thesaurus 1. Represent the query in the concept space by using the representation of the index terms 2. Compute the similarity sim(q,kv) between each term kv and the whole query 3. Expand the query with the top r ranked terms according to sim(q,kv)

22. Example of Similarity Thesaurus The distance of a given term kv to the query centroid QC might be quite distinct from the distances of kv to the individual query terms ki QC={ka ,kb} kv kj ka QC kb

23. Query Expansion based on a Similarity Thesaurus • A document dj is represented term-concept space by • If the original query q is expanded to include all the t index terms, then the similarity sim(q, dj) between the document dj and the query q can be computed as • which is similar to the generalized vector space model

24. Query Expansion based on a Statistical Thesaurus • Idea by Crouch and Yang (1992) • Use complete link algorithm to produce small and tight clusters • Use term discrimination value to select terms for entry into a particular thesaurus class • Term discrimination value • A measure of the change in space separation which occurs when a given term is assigned to the document collection

25. Term Discrimination Value • Terms • good discriminators: (terms with positive discrimination values) • index terms • indifferent discriminators: (near-zero discrimination values) • thesaurus class • poor discriminators: (negative discrimination values) • term phrases • Document frequency dfk • dfk >n/10: high frequency term (poor discriminators) • dfk <n/100: low frequency term (indifferent discriminators) • n/100  dfkn/10: good discriminator

26. Statistical Thesaurus • Term discrimination value theory • the terms which make up a thesaurus class must be indifferent discriminators • The proposed approach • cluster the document collection into small, tight clusters • A thesaurus class is defined as the intersection of all the low frequency terms in that cluster • documents are indexed by the thesaurus classes • the thesaurus classes are weighted by

27. Discussion • Query expansion • useful • little explored technique • Trends and research issues • The combination of local analysis, global analysis, visual displays, and interactive interfaces is also a current and important research problem