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Recall the basic indexing pipeline

Recall the basic indexing pipeline
Introduction to Information Retrieval Introduction to Information Retrieval Document ingestion www.ThesisScientist.comIntroduction to Information Retrieval Recall the basic indexing pipeline Documents to Friends, Romans, countrymen. be indexed Tokenizer Token stream Friends Romans Countrymen Linguistic modules friend roman countryman Modified tokens 2 4 Indexer friend 1 2 roman Inverted index 16 13 countryman www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.1 Parsing a document  What format is it in  pdf/word/excel/html  What language is it in  What character set is in use  (CP1252, UTF8, …) Each of these is a classification problem, which we will study later in the course. But these tasks are often done heuristically … www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.1 Complications: Format/language  Documents being indexed can include docs from many different languages  A single index may contain terms from many languages.  Sometimes a document or its components can contain multiple languages/formats  French email with a German pdf attachment.  French email quote clauses from an Englishlanguage contract  There are commercial and open source libraries that can handle a lot of this stuff www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.1 Complications: What is a document We return from our query “documents” but there are often interesting questions of grain size: What is a unit document  A file  An email (Perhaps one of many in a single mbox file)  What about an email with 5 attachments  A group of files (e.g., PPT or LaTeX split over HTML pages) www.ThesisScientist.comIntroduction to Information Retrieval Introduction to Information Retrieval Tokens www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.1 Tokenization  Input: “Friends, Romans and Countrymen”  Output: Tokens  Friends  Romans  Countrymen  A token is an instance of a sequence of characters  Each such token is now a candidate for an index entry, after further processing  Described below  But what are valid tokens to emit www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.1 Tokenization  Issues in tokenization:  Finland’s capital  Finland AND s Finlands Finland’s  HewlettPackard Hewlett and Packard as two tokens  stateoftheart: break up hyphenated sequence.  coeducation  lowercase, lowercase, lower case  It can be effective to get the user to put in possible hyphens  San Francisco: one token or two  How do you decide it is one token www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.1 Numbers  3/20/91 Mar. 12, 1991 20/3/91  55 B.C.  B52  My PGP key is 324a3df234cb23e  (800) 2342333  Often have embedded spaces  Older IR systems may not index numbers  But often very useful: think about things like looking up error codes/stacktraces on the web  (One answer is using ngrams: IIR ch. 3)  Will often index “metadata” separately  Creation date, format, etc. www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.1 Tokenization: language issues  French  L'ensemble one token or two  L L’ Le  Want l’ensemble to match with un ensemble  Until at least 2003, it didn’t on Google  Internationalization  German noun compounds are not segmented  Lebensversicherungsgesellschaftsangestellter  ‘life insurance company employee’  German retrieval systems benefit greatly from a compound splitter module  Can give a 15 performance boost for German www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.1 Tokenization: language issues  Chinese and Japanese have no spaces between words: 莎拉波娃现在居住在美国东南部的佛罗里达。  Not always guaranteed a unique tokenization  Further complicated in Japanese, with multiple alphabets intermingled  Dates/amounts in multiple formats フォーチュン500社は情報不足のため時間あた500K(約6,000万円) Katakana Hiragana Kanji Romaji Enduser can express query entirely in hiragana www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.1 Tokenization: language issues  Arabic (or Hebrew) is basically written right to left, but with certain items like numbers written left to right  Words are separated, but letter forms within a word form complex ligatures  ← → ← → ← start ‘Algeria achieved its independence in 1962 after 132 years of French occupation.’  With Unicode, the surface presentation is complex, but the stored form is straightforward www.ThesisScientist.comIntroduction to Information Retrieval Introduction to Information Retrieval Terms The things indexed in an IR system www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.2 Stop words  With a stop list, you exclude from the dictionary entirely the commonest words. Intuition:  They have little semantic content: the, a, and, to, be  There are a lot of them: 30 of postings for top 30 words  But the trend is away from doing this:  Good compression techniques (IIR 5) means the space for including stop words in a system is very small  Good query optimization techniques (IIR 7) mean you pay little at query time for including stop words.  You need them for:  Phrase queries: “King of Denmark”  Various song titles, etc.: “Let it be”, “To be or not to be”  “Relational” queries: “flights to London” www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.3 Normalization to terms  We may need to “normalize” words in indexed text as well as query words into the same form  We want to match U.S.A. and USA  Result is terms: a term is a (normalized) word type, which is an entry in our IR system dictionary  We most commonly implicitly define equivalence classes of terms by, e.g.,  deleting periods to form a term  U.S.A., USA  USA  deleting hyphens to form a term  antidiscriminatory, antidiscriminatory  antidiscriminatory www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.3 Normalization: other languages  Accents: e.g., French résumé vs. resume.  Umlauts: e.g., German: Tuebingen vs. Tübingen  Should be equivalent  Most important criterion:  How are your users like to write their queries for these words  Even in languages that standardly have accents, users often may not type them  Often best to normalize to a deaccented term  Tuebingen, Tübingen, Tubingen  Tubingen www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.3 Normalization: other languages  Normalization of things like date forms  7月30日 vs. 7/30  Japanese use of kana vs. Chinese characters  Tokenization and normalization may depend on the language and so is intertwined with language detection Is this German “mit” Morgen will ich in MIT …  Crucial: Need to “normalize” indexed text as well as query terms identically www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.3 Case folding  Reduce all letters to lower case  exception: upper case in midsentence  e.g., General Motors  Fed vs. fed  SAIL vs. sail  Often best to lower case everything, since users will use lowercase regardless of ‘correct’ capitalization…  Longstanding Google example: fixed in 2011…+  Query C.A.T.  1 result is for “cats” (well, Lolcats) not Caterpillar Inc. www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.3 Normalization to terms  An alternative to equivalence classing is to do asymmetric expansion  An example of where this may be useful  Enter: window Search: window, windows  Enter: windows Search: Windows, windows, window  Enter: Windows Search: Windows  Potentially more powerful, but less efficient www.ThesisScientist.comIntroduction to Information Retrieval Thesauri and soundex  Do we handle synonyms and homonyms  E.g., by handconstructed equivalence classes  car = automobile color = colour  We can rewrite to form equivalenceclass terms  When the document contains automobile, index it under car automobile (and viceversa)  Or we can expand a query  When the query contains automobile, look under car as well  What about spelling mistakes  One approach is Soundex, which forms equivalence classes of words based on phonetic heuristics  More in IIR 3 and IIR 9 www.ThesisScientist.comIntroduction to Information Retrieval Introduction to Information Retrieval Stemming and Lemmatization www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.4 Lemmatization  Reduce inflectional/variant forms to base form  E.g.,  am, are, is  be  car, cars, car's, cars' car  the boy's cars are different colors the boy car be different color  Lemmatization implies doing “proper” reduction to dictionary headword form www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.4 Stemming  Reduce terms to their “roots” before indexing  “Stemming” suggests crude affix chopping  language dependent  e.g., automate(s), automatic, automation all reduced to automat. for exampl compress and for example compressed compress ar both accept and compression are both as equival to compress accepted as equivalent to compress. www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.4 Porter’s algorithm  Commonest algorithm for stemming English  Results suggest it’s at least as good as other stemming options  Conventions + 5 phases of reductions  phases applied sequentially  each phase consists of a set of commands  sample convention: Of the rules in a compound command, select the one that applies to the longest suffix. www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.4 Typical rules in Porter  sses ss  ies i  ational ate  tional tion  Weight of word sensitive rules  (m1) EMENT →  replacement → replac  cement → cement www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.4 Other stemmers  Other stemmers exist:  Lovins stemmer  http://www.comp.lancs.ac.uk/computing/research/stemming/general/lovins.htm  Singlepass, longest suffix removal (about 250 rules)  Paice/Husk stemmer  Snowball  Full morphological analysis (lemmatization)  At most modest benefits for retrieval www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.4 Languagespecificity  The above methods embody transformations that are  Languagespecific, and often  Applicationspecific  These are “plugin” addenda to the indexing process  Both open source and commercial plugins are available for handling these www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.2.4 Does stemming help  English: very mixed results. Helps recall for some queries but harms precision on others  E.g., operative (dentistry) ⇒ oper  Definitely useful for Spanish, German, Finnish, …  30 performance gains for Finnish www.ThesisScientist.comIntroduction to Information Retrieval Introduction to Information Retrieval Faster postings merges: Skip pointers/Skip lists www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.3 Recall basic merge  Walk through the two postings simultaneously, in time linear in the total number of postings entries 2 4 8 41 48 64 128 Brutus 2 8 1 2 3 8 11 17 21 31 Caesar If the list lengths are m and n, the merge takes O(m+n) operations. Can we do better Yes (if the index isn’t changing too fast). www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.3 Augment postings with skip pointers (at indexing time) 128 41 2 4 8 41 48 64 128 31 11 1 2 3 8 11 17 21 31  Why  To skip postings that will not figure in the search results.  How  Where do we place skip pointers www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.3 Query processing with skip pointers 128 41 2 4 8 41 48 64 128 31 11 1 2 3 8 11 17 21 31 Suppose we’ve stepped through the lists until we process 8 on each list. We match it and advance. We then have 41 and 11 on the lower. 11 is smaller. But the skip successor of 11 on the lower list is 31, so we can skip ahead past the intervening postings. www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.3 Where do we place skips  Tradeoff:  More skips  shorter skip spans  more likely to skip. But lots of comparisons to skip pointers.  Fewer skips  few pointer comparison, but then long skip spans  few successful skips. www.ThesisScientist.comIntroduction to Information Retrieval Sec. 2.3 Placing skips  Simple heuristic: for postings of length L, use L evenlyspaced skip pointers Moffat and Zobel 1996  This ignores the distribution of query terms.  Easy if the index is relatively static; harder if L keeps changing because of updates.  This definitely used to help; with modern hardware it may not unless you’re memorybased Bahle et al. 2002  The I/O cost of loading a bigger postings list can outweigh the gains from quicker in memory merging www.ThesisScientist.com
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