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Introducing Information Retrieval and Web Search

Introducing Information Retrieval and Web Search
Introduction to Information Retrieval Introducing Information Retrieval and Web Search www.ThesisScientist.comInformation Retrieval • Information Retrieval (IR) is finding material (usually documents) of an unstructured nature (usually text) that satisfies an information need from within large collections (usually stored on computers). – These days we frequently think first of web search, but there are many other cases: • Email search • Searching your laptop • Corporate knowledge bases • Legal information retrieval www.ThesisScientist.comUnstructured (text) vs. structured (database) data in the midnineties www.ThesisScientist.comUnstructured (text) vs. structured (database) data today www.ThesisScientist.comSec. 1.1 Basic assumptions of Information Retrieval • Collection: A set of documents – Assume it is a static collection for the moment • Goal: Retrieve documents with information that is relevant to the user’s information need and helps the user complete a task www.ThesisScientist.comThe classic search model Get rid of mice in a User task politically correct way Misconception Info about removing mice Info need without killing them Misformulation Query Searc how trap mice alive h Search engine Query Results Collection refinement www.ThesisScientist.comSec. 1.1 How good are the retrieved docs  Precision : Fraction of retrieved docs that are relevant to the user’s information need  Recall : Fraction of relevant docs in collection that are retrieved  More precise definitions and measurements to follow later www.ThesisScientist.comIntroduction to Information Retrieval Termdocument incidence matrices www.ThesisScientist.comSec. 1.1 Unstructured data in 1620 • Which plays of Shakespeare contain the words Brutus AND Caesar but NOT Calpurnia • One could grep all of Shakespeare’s plays for Brutus and Caesar, then strip out lines containing Calpurnia • Why is that not the answer – Slow (for large corpora) – NOT Calpurnia is nontrivial – Other operations (e.g., find the word Romans near countrymen) not feasible – Ranked retrieval (best documents to return) • Later lectures www.ThesisScientist.comSec. 1.1 Termdocument incidence matrices Antony and Cleopatra Julius Caesar The Tempest Hamlet Othello Macbeth Antony 1 1 0 0 0 1 Brutus 1 1 0 1 0 0 Caesar 1 1 0 1 1 1 Calpurnia 0 1 0 0 0 0 Cleopatra 1 0 0 0 0 0 mercy 1 0 1 1 1 1 worser 1 0 1 1 1 0 1 if play contains Brutus AND Caesar BUT NOT word, 0 otherwise Calpurnia www.ThesisScientist.comSec. 1.1 Incidence vectors • So we have a 0/1 vector for each term. • To answer query: take the vectors for Brutus, Caesar and Calpurnia (complemented)  bitwise AND. – 110100 AND Antony and Cleopatra Julius Caesar The Tempest Hamlet Othello Macbeth – 110111 AND Antony 1 1 0 0 0 1 Brutus 1 1 0 1 0 0 Caesar 1 1 0 1 1 1 – 101111 = Calpurnia 0 1 0 0 0 0 Cleopatra 1 0 0 0 0 0 mercy 1 0 1 1 1 1 – 100100 worser 1 0 1 1 1 0 www.ThesisScientist.comSec. 1.1 Answers to query • Antony and Cleopatra, Act III, Scene ii Agrippa Aside to DOMITIUS ENOBARBUS: Why, Enobarbus, When Antony found Julius Caesar dead, He cried almost to roaring; and he wept When at Philippi he found Brutus slain. • Hamlet, Act III, Scene ii Lord Polonius: I did enact Julius Caesar I was killed i’ the Capitol; Brutus killed me. www.ThesisScientist.comSec. 1.1 Bigger collections • Consider N = 1 million documents, each with about 1000 words. • Avg 6 bytes/word including spaces/punctuation – 6GB of data in the documents. • Say there are M = 500K distinct terms among these. www.ThesisScientist.comSec. 1.1 Can’t build the matrix • 500K x 1M matrix has halfatrillion 0’s and 1’s. Why • But it has no more than one billion 1’s. – matrix is extremely sparse. • What’s a better representation – We only record the 1 positions. www.ThesisScientist.comIntroduction to Information Retrieval The Inverted Index The key data structure underlying modern IR www.ThesisScientist.comSec. 1.2 Inverted index • For each term t, we must store a list of all documents that contain t. – Identify each doc by a docID, a document serial number • Can we used fixedsize arrays for this Brutus 1 2 4 11 31 45173174 Caesar 1 2 4 5 6 16 57 132 Calpurnia 2 31 54101 What happens if the word Caesar is added to document 14 www.ThesisScientist.comSec. 1.2 Inverted index • We need variablesize postings lists – On disk, a continuous run of postings is normal and best Posting – In memory, can use linked lists or variable length arrays Brutus 1 2 4 11 31 45173174 • Some tradeoffs in size/ease of insertion Caesar 1 2 4 5 6 16 57 132 Calpurnia 2 31 54101 Dictionary Postings www.ThesisScientist.com Sorted by docID (more later on why).Sec. 1.2 Inverted index construction Documents to Friends, Romans, countrymen. be indexed Tokenizer Friends Token stream Romans Countrymen Linguistic modules friend roman countryman Modified tokens Indexer friend 2 4 roman 1 2 Inverted index countryman 16 www.ThesisScientist.com 13Initial stages of text processing • Tokenization – Cut character sequence into word tokens • Deal with “John’s”, a stateoftheart solution • Normalization – Map text and query term to same form • You want U.S.A. and USA to match • Stemming – We may wish different forms of a root to match • authorize, authorization • Stop words – We may omit very common words (or not) • the, a, to, of www.ThesisScientist.comSec. 1.2 Indexer steps: Token sequence • Sequence of (Modified token, Document ID) pairs. Doc 1 Doc 2 I did enact Julius So let it be with Caesar I was killed Caesar. The noble i’ the Capitol; Brutus hath told you Brutus killed me. Caesar was ambitious www.ThesisScientist.comSec. 1.2 Indexer steps: Sort • Sort by terms – And then docID Core indexing step www.ThesisScientist.comSec. 1.2 Indexer steps: Dictionary Postings • Multiple term entries in a single document are merged. • Split into Dictionary and Postings • Doc. frequency information is added. Why frequency Will discuss later. www.ThesisScientist.comSec. 1.2 Where do we pay in storage Lists of docIDs Terms and counts IR system implementation • How do we index efficiently • How much storage do we need Pointers www.ThesisScientist.comIntroduction to Information Retrieval Query processing with an inverted index www.ThesisScientist.comSec. 1.3 The index we just built • How do we process a query Our focus – Later what kinds of queries can we process www.ThesisScientist.comSec. 1.3 Query processing: AND • Consider processing the query: Brutus AND Caesar – Locate Brutus in the Dictionary; • Retrieve its postings. – Locate Caesar in the Dictionary; • Retrieve its postings. – “Merge” the two postings (intersect the document 2 4 8 16 32 64 128 Brutus sets): Caesar 1 2 3 5 8 13 21 34 www.ThesisScientist.comSec. 1.3 The merge • Walk through the two postings simultaneously, in time linear in the total number of postings entries 2 4 8 16 128 32 64 Brutus Caesar 1 2 5 8 13 21 34 3 If the list lengths are x and y, the merge takes O(x+y) operations. Crucial: postings sorted by docID. www.ThesisScientist.comIntersecting two postings lists (a “merge” algorithm) www.ThesisScientist.comIntroduction to Information Retrieval The Boolean Retrieval Model Extended Boolean Models www.ThesisScientist.comSec. 1.3 Boolean queries: Exact match • The Boolean retrieval model is being able to ask a query that is a Boolean expression: – Boolean Queries are queries using AND, OR and NOT to join query terms • Views each document as a set of words • Is precise: document matches condition or not. – Perhaps the simplest model to build an IR system on • Primary commercial retrieval tool for 3 decades. • Many search systems you still use are Boolean: – Email, library catalog, Mac OS X Spotlight www.ThesisScientist.comSec. 1.4 Example: WestLaw http://www.westlaw.com/ • Largest commercial (paying subscribers) legal search service (started 1975; ranking added 1992; new federated search added 2010) • Tens of terabytes of data; 700,000 users • Majority of users still use boolean queries • Example query: – What is the statute of limitations in cases involving the federal tort claims act – LIMIT /3 STATUTE ACTION /S FEDERAL /2 TORT /3 CLAIM • /3 = within 3 words, /S = in same sentence www.ThesisScientist.comSec. 1.4 Example: WestLaw http://www.westlaw.com/ • Another example query: – Requirements for disabled people to be able to access a workplace – disabl /p access /s worksite workplace (employment /3 place • Note that SPACE is disjunction, not conjunction • Long, precise queries; proximity operators; incrementally developed; not like web search • Many professional searchers still like Boolean search – You know exactly what you are getting • But that doesn’t mean it actually works better…. www.ThesisScientist.comSec. 1.3 Boolean queries: More general merges • Exercise: Adapt the merge for the queries: Brutus AND NOT Caesar Brutus OR NOT Caesar • Can we still run through the merge in time O(x+y) What can we achieve www.ThesisScientist.comSec. 1.3 Merging What about an arbitrary Boolean formula (Brutus OR Caesar) AND NOT (Antony OR Cleopatra) • Can we always merge in “linear” time – Linear in what • Can we do better www.ThesisScientist.comSec. 1.3 Query optimization • What is the best order for query processing • Consider a query that is an AND of n terms. • For each of the n terms, get its postings, Brutus 2 4 8 16 32 64128 then AND them together. Caesar 1 2 3 5 8 16 21 34 Calpurnia 13 16 Query: Brutus AND Calpurnia AND Caesar 35 www.ThesisScientist.comSec. 1.3 Query optimization example • Process in order of increasing freq: – start with smallest set, then keep cutting further. This is why we kept document freq. in dictionary Brutus 2 4 8 16 32 64128 Caesar 1 2 3 5 8 16 21 34 Calpurnia 13 16 Execute the query as (Calpurnia AND Brutus) AND Caesar. www.ThesisScientist.comSec. 1.3 More general optimization • e.g., (madding OR crowd) AND (ignoble OR strife) • Get doc. freq.’s for all terms. • Estimate the size of each OR by the sum of its doc. freq.’s (conservative). • Process in increasing order of OR sizes. www.ThesisScientist.comExercise • Recommend a query processing order for (tangerine OR trees) AND Term Freq (marmalade OR skies) AND eyes 213312 (kaleidoscope OR eyes) kaleidoscope 87009 marmalade 107913 • Which two terms should we skies 271658 process first tangerine 46653 trees 316812 www.ThesisScientist.comQuery processing exercises • Exercise: If the query is friends AND romans AND (NOT countrymen), how could we use the freq of countrymen • Exercise: Extend the merge to an arbitrary Boolean query. Can we always guarantee execution in time linear in the total postings size • Hint: Begin with the case of a Boolean formula query: in this, each query term appears only once in the query. www.ThesisScientist.comExercise • Try the search feature at http://www.rhymezone.com/shakespeare/ • Write down five search features you think it could do better www.ThesisScientist.comIntroduction to Information Retrieval Phrase queries and positional indexes www.ThesisScientist.comSec. 2.4 Phrase queries • We want to be able to answer queries such as “stanford university” – as a phrase • Thus the sentence “I went to university at Stanford” is not a match. – The concept of phrase queries has proven easily understood by users; one of the few “advanced search” ideas that works – Many more queries are implicit phrase queries • For this, it no longer suffices to store only term : docs entries www.ThesisScientist.comSec. 2.4.1 A first attempt: Biword indexes • Index every consecutive pair of terms in the text as a phrase • For example the text “Friends, Romans, Countrymen” would generate the biwords – friends romans – romans countrymen • Each of these biwords is now a dictionary term • Twoword phrase queryprocessing is now immediate. www.ThesisScientist.comSec. 2.4.1 Longer phrase queries • Longer phrases can be processed by breaking them down • stanford university palo alto can be broken into the Boolean query on biwords: stanford university AND university palo AND palo alto Without the docs, we cannot verify that the docs matching the above Boolean query do contain the phrase. Can have false positives www.ThesisScientist.comSec. 2.4.1 Issues for biword indexes • False positives, as noted before • Index blowup due to bigger dictionary – Infeasible for more than biwords, big even for them • Biword indexes are not the standard solution (for all biwords) but can be part of a compound strategy www.ThesisScientist.comSec. 2.4.2 Solution 2: Positional indexes • In the postings, store, for each term the position(s) in which tokens of it appear: term, number of docs containing term; doc1: position1, position2 … ; doc2: position1, position2 … ; etc. www.ThesisScientist.comSec. 2.4.2 Positional index example be: 993427; 1: 7, 18, 33, 72, 86, 231; Which of docs 1,2,4,5 2: 3, 149; could contain “to be or not to be” 4: 17, 191, 291, 430, 434; 5: 363, 367, … • For phrase queries, we use a merge algorithm recursively at the document level • But we now need to deal with more than just equality www.ThesisScientist.comSec. 2.4.2 Processing a phrase query • Extract inverted index entries for each distinct term: to, be, or, not. • Merge their doc:position lists to enumerate all positions with “to be or not to be”. – to: • 2:1,17,74,222,551; 4:8,16,190,429,433; 7:13,23,191; ... – be: • 1:17,19; 4:17,191,291,430,434; 5:14,19,101; ... • Same general method for proximity searches www.ThesisScientist.comSec. 2.4.2 Proximity queries • LIMIT /3 STATUTE /3 FEDERAL /2 TORT – Again, here, /k means “within k words of”. • Clearly, positional indexes can be used for such queries; biword indexes cannot. • Exercise: Adapt the linear merge of postings to handle proximity queries. Can you make it work for any value of k – This is a little tricky to do correctly and efficiently – See Figure 2.12 of IIR www.ThesisScientist.comSec. 2.4.2 Positional index size • A positional index expands postings storage substantially – Even though indices can be compressed • Nevertheless, a positional index is now standardly used because of the power and usefulness of phrase and proximity queries … whether used explicitly or implicitly in a ranking retrieval system. www.ThesisScientist.comSec. 2.4.2 Positional index size • Need an entry for each occurrence, not just once per document Why • Index size depends on average document size – Average web page has 1000 terms – SEC filings, books, even some epic poems … easily 100,000 terms • Consider a term with frequency 0.1 Document size Positional postings Postings 1000 1 1 100,000 1 100 www.ThesisScientist.comSec. 2.4.2 Rules of thumb • A positional index is 2–4 as large as a non positional index • Positional index size 35–50 of volume of original text – Caveat: all of this holds for “Englishlike” languages www.ThesisScientist.comSec. 2.4.3 Combination schemes • These two approaches can be profitably combined – For particular phrases (“Michael Jackson”, “Britney Spears”) it is inefficient to keep on merging positional postings lists • Even more so for phrases like “The Who” • Williams et al. (2004) evaluate a more sophisticated mixed indexing scheme – A typical web query mixture was executed in ¼ of the time of using just a positional index – It required 26 more space than having a positional index alone www.ThesisScientist.comIntroduction to Information Retrieval Structured vs. Unstructured Data www.ThesisScientist.comIR vs. databases: Structured vs unstructured data • Structured data tends to refer to information in “tables” Employee Manager Salary Smith Jones 50000 Chang Smith 60000 Ivy Smith 50000 Typically allows numerical range and exact match (for text) queries, e.g., Salary 60000 AND Manager = Smith. www.ThesisScientist.comUnstructured data • Typically refers to free text • Allows – Keyword queries including operators – More sophisticated “concept” queries e.g., • find all web pages dealing with drug abuse • Classic model for searching text documents www.ThesisScientist.comSemistructured data • In fact almost no data is “unstructured” • E.g., this slide has distinctly identified zones such as the Title and Bullets • … to say nothing of linguistic structure • Facilitates “semistructured” search such as – Title contains data AND Bullets contain search • Or even – Title is about Object Oriented Programming AND Author something like strorup – where is the wildcard operator www.ThesisScientist.com