Information Retrieval and Web Search

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Information Retrieval and Web Search

• Heng Ji
• hengji_at_cs.qc.cuny.edu
• Sept 16, 2008

Acknowledgement some slides from Jimmy Lin,
Victor Lavrenko
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Outline

• Introduction
• IR Approaches and Ranking
• Query Construction
• Document Indexing
• Web Search
• Project Topic Discussion and Finalization

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What is Information Retrieval?

• Most people equate IR with web-search
• highly visible, commercially successful endeavors
• leverage 3 decades of academic research
• IR finding any kind of relevant information
• web-pages, news events, answers, images,
• relevance is a key notion

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IR System
IR System
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What types of information?

• Text (Documents and portions thereof)
• XML and structured documents
• Images
• Audio (sound effects, songs, etc.)
• Video
• Source code
• Applications/Web services

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Interesting Examples

• Google image search
• Google video search
• NYU Prof. Sekines Ngram search
• http//linserv1.cims.nyu.edu23232/ngram/
• INDRI Demo Show
• http//www.lemurproject.org/indri/

http//images.google.com/
http//video.google.com/
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What about databases?

• What are examples of databases?
• Banks storing account information
• Retailers storing inventories
• Universities storing student grades
• What exactly is a (relational) database?
• Think of them as a collection of tables
• They model some aspect of the world

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A (Simple) Database Example
Student Table
Department Table
Course Table
Enrollment Table
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Database Queries

• What would you want to know from a database?
• What classes is John Arrow enrolled in?
• Who has the highest grade in LBSC 690?
• Whos in the history department?
• Of all the non-CLIS students taking LBSC 690 with
  a last name shorter than six characters and were
  born on a Monday, who has the longest email
  address?

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Comparing IR to databases
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The IR Black Box
Documents
Query
Hits
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Inside The IR Black Box
Documents
Query
Representation Function
Representation Function
Query Representation
Document Representation
Index
Comparison Function
Hits
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Building the IR Black Box

• Different models of information retrieval
• Boolean model
• Vector space model
• Languages models
• Representing the meaning of documents
• How do we capture the meaning of documents?
• Is meaning just the sum of all terms?
• Indexing
• How do we actually store all those words?
• How do we access indexed terms quickly?

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Outline

• Introduction
• IR Approaches and Ranking
• Query Construction
• Document Indexing
• Web Search

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The Central Problem in IR
Information Seeker
Authors
Concepts
Concepts
Query Terms
Document Terms
Do these represent the same concepts?
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Relevance

• Relevance is a subjective judgment and may
  include
• Being on the proper subject.
• Being timely (recent information).
• Being authoritative (from a trusted source).
• Satisfying the goals of the user and his/her
  intended use of the information (information
  need).

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IR Ranking

• Early IR focused on set-based retrieval
• Boolean queries, set of conditions to be
  satisfied
• document either matches the query or not
• like classifying the collection into relevant /
  non-relevant sets
• still used by professional searchers
• advanced search in many systems
• Modern IR ranked retrieval
• free-form query expresses users information need
• rank documents by decreasing likelihood of
  relevance
• many studies prove it is superior

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A heuristic formula for IR

• Rank docs by similarity to the query
• suppose the query is cryogenic labs
• Similarity query words in the doc
• favors documents with both labs and cryogenic
• mathematically
• Logical variations (set-based)
• Boolean AND (require all words)
• Boolean OR (any of the words)

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Term Frequency (TF)

• Observation
• key words tend to be repeated in a document
• Modify our similarity measure
• give more weight if word occurs multiple times
• Problem
• biased towards long documents
• spurious occurrences
• normalize by length

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Inverse Document Frequency (IDF)

• Observation
• rare words carry more meaning cryogenic, apollo
• frequent words are linguistic glue of, the,
  said, went
• Modify our similarity measure
• give more weight to rare words but dont be
  too aggressive (why?)
• C total number of documents
• df(q) total number of documents that contain q

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TF normalization

• Observation
• D1cryogenic,labs, D2 cryogenic,cryogenic
• which document is more relevant?
• which one is ranked higher? (df(labs) gt
  df(cryogenic))
• Correction
• first occurrence more important than a repeat
  (why?)
• squash the linearity of TF

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State-of-the-art Formula
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Vector-space approach to IR
cat

• cat cat

• cat pig

pig

• pig cat

dog
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Language-modeling Approach

• query is a random sample from a perfect
  document
• words are sampled independently of each other
• rank documents by the probability of generating
  query

D
query
4/9 2/9 4/9 3/9
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PageRank in Google
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PageRank in Google (Cont)
I1
A
B
I2

• Assign a numeric value to each page
• The more a page is referred to by important
  pages, the more this page is important
• d damping factor (0.85)
• Many other criteria e.g. proximity of query
  words
• information retrieval better than
  information retrieval

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Outline

• Introduction
• IR Approaches and Ranking
• Query Construction
• Document Indexing
• Web Search

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Keyword Search

• Simplest notion of relevance is that the query
  string appears verbatim in the document.
• Slightly less strict notion is that the words in
  the query appear frequently in the document, in
  any order (bag of words).

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Problems with Keywords

• May not retrieve relevant documents that include
  synonymous terms.
• restaurant vs. café
• PRC vs. China
• May retrieve irrelevant documents that include
  ambiguous terms.
• bat (baseball vs. mammal)
• Apple (company vs. fruit)
• bit (unit of data vs. act of eating)

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Query Expansion

• http//www.lemurproject.org/lemur/IndriQueryLangua
  ge.php
• Most errors caused by vocabulary mismatch
• query cars, document automobiles
• solution automatically add highly-related words
• Thesaurus / WordNet lookup
• add semantically-related words (synonyms)
• cannot take context into account
• rail car vs. race car vs. car and cdr
• Statistical Expansion
• add statistically-related words (co-occurrence)
• very successful

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IR Query Examples

• http//nlp.cs.qc.cuny.edu/ir.zip
• Query
• ltparametersgtltquerygtcombine( weight( 0.063356
  1(explosion) 0.187417 1(blast) 0.411817
  1(wounded) 0.101370 1(injured) 0.161191
  1(death) 0.074849 1(deaths)) weight( 0.311760
  1(Davao Cityinternational airport) 0.311760
  1(Tuesday) 0.103044 1(DAVAO) 0.195505
  1(Philippines) 0.019817 1(DXDC) 0.058113
  1(Davao Medical Center)))lt/querygtlt/parametersgt

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Outline

• Introduction
• IR Approaches and Ranking
• Query Construction
• Document Indexing
• Web Search

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Document indexing

• Goal Find the important meanings and create an
  internal representation
• Factors to consider
• Accuracy to represent meanings (semantics)
• Exhaustiveness (cover all the contents)
• Facility for computer to manipulate
• What is the best representation of contents?
• Char. string (char trigrams) not precise enough
• Word good coverage, not precise
• Phrase poor coverage, more precise
• Concept poor coverage, precise

Accuracy (Precision)
Coverage (Recall)
String Word Phrase Concept
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Indexer steps

• Sequence of (Modified token, Document ID) pairs.

Doc 1
Doc 2
I did enact Julius Caesar I was killed i' the
Capitol Brutus killed me.
So let it be with Caesar. The noble Brutus hath
told you Caesar was ambitious
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• Multiple term entries in a single document are
  merged.
• Frequency information is added.

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Stopwords / Stoplist

• function words do not bear useful information for
  IR
• of, in, about, with, I, although,
• Stoplist contain stopwords, not to be used as
  index
• Prepositions
• Articles
• Pronouns
• Some adverbs and adjectives
• Some frequent words (e.g. document)
• The removal of stopwords usually improves IR
  effectiveness
• A few standard stoplists are commonly used.

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Stemming

• Reason
• Different word forms may bear similar meaning
  (e.g. search, searching) create a standard
  representation for them
• Stemming
• Removing some endings of word
• computer
• compute
• computes
• computing
• computed
• computation

comput
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Lemmatization

• transform to standard form according to syntactic
  category.
• E.g. verb ing ? verb
• noun s ? noun
• Need POS tagging
• More accurate than stemming, but needs more
  resources
• crucial to choose stemming/lemmatization rules
• noise v.s. recognition rate
• compromise between precision and recall
• light/no stemming severe stemming
• -recall precision recall -precision

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Outline

• Introduction
• IR Approaches and Ranking
• Query Construction
• Document Indexing
• Web Search

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IR on the Web

• No stable document collection (spider, crawler)
• Invalid document, duplication, etc.
• Huge number of documents (partial collection)
• Multimedia documents
• Great variation of document quality
• Multilingual problem

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Web Search

• Application of IR to HTML documents on the World
  Wide Web.
• Differences
• Must assemble document corpus by spidering the
  web.
• Can exploit the structural layout information in
  HTML (XML).
• Documents change uncontrollably.
• Can exploit the link structure of the web.

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Web Search System
IR System
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• Technical Backup

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Some formulas for Sim

• Dot product
• Cosine
• Dice
• Jaccard

t1
D
Q
t2