Lecture 2: The term vocabulary and postings lists Related to Chapter 2:

1. Lecture 2: The term vocabulary and postings lists Related to Chapter 2: http://nlp.stanford.edu/IR-book/pdf/02voc.pdf

2. Ch. 1 Recap of the previous lecture • Basic inverted indexes: • Structure: Dictionary and Postings • Boolean query processing • Intersection by linear time “merging” • Simple optimizations

3. Plan for this lecture • Preprocessing to form the term vocabulary • Tokenization • Normalization • Postings • Faster merges: skip lists • Positional postings and phrase queries

4. Tokenizer Friends Romans Countrymen Token stream. Linguistic modules friend Modified tokens. friend roman countryman roman Indexer 2 4 countryman 1 2 Inverted index. 16 13 Recall the basicindexing pipeline Documents to be indexed. Friends, Romans, countrymen.

5. Sec. 2.1 The first step: Parsing a document • What format is it in? • pdf/word/excel/html • What language is it in? • What character set is in use?

6. Sec. 2.1 Complications: Format/language • Documents being indexed can include docs from many different languages • A single index may have to contain terms of several languages. • Sometimes a document or its components can contain multiple languages/formats • French email with a German pdf attachment.

7. Tokenization

8. Tokenization • Given a character sequence, tokenization is the task of chopping it up into pieces, called tokens. • Perhaps at the same time throwing away certain characters, such as punctuation.

9. Sec. 2.2.1 Tokenization • Input: “university of Qom, computer department” • Output: Tokens • university • of • Qom • computer • Department • Each such token is now a candidate for an index entry, after further processing: Normalization. • But what are valid tokens to emit?

10. Sec. 2.2.1 Issues in tokenization • Iran’s capital  Iran? Irans? Iran’s? • Hyphen • Hewlett-Packard  Hewlett and Packard as two tokens? • the hold-him-back-and-drag-him-away maneuver • co-author • lowercase, lower-case, lower case ? • Space • San Francisco: How do you decide it is one token?

11. Sec. 2.2.1 Issues in tokenization • Numbers • Older IR systems may not index numbers • But often very useful: looking up error codes/stack traces on the web • 3/12/91 Mar. 12, 1991 12/3/91 • 55 B.C. • B-52 • My PGP key is 324a3df234cb23e • (800) 234-2333

12. Sec. 2.2.1 Language issues in tokenization • 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

13. Sec. 2.2.1 Katakana Hiragana Kanji Romaji Language issues in tokenization • 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。 • Further complicated in Japanese, with multiple alphabets intermingled 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)

14. Sec. 2.2.2 Stop words • With a stop list, you exclude from the dictionary words like the, a, and, to, be • Intuition: They have little semantic content. • The commonest words. • Using a stop list significantly reduces the number of postings that a system has to store, because there are a lot of them. • Two ways for construction: • By expert • By machine

15. Stop words • But you need them for: • Phrase queries: “President of Iran” • Various song titles, etc.: “Let it be”, “To be or not to be” • “Relational” queries: “flights to London” • The general trend in IR systems: from large stop lists (200–300 terms) to very small stop lists (7–12 terms) to no stop list. • Good compression techniques (lecture 5) means the space for including stop words in a system is very small • Good query optimization techniques (lecture 7) mean you pay little at query time for including stop words

16. normalization

17. Sec. 2.2.3 Normalization to terms • Example: We want to match I.R. and IR • Token normalization is the process of canonicalizingtokens so that matches occur despite superficial differences in the character sequences of the tokens • Result is terms: • A term is a (normalized) word type, which is an entry in our IR system dictionary

18. Sec. 2.2.3 Normalization Example: Case folding • Reduce all letters to lower case • Exception: upper case in mid-sentence? • Often best to lower case everything, since users will use lowercase regardless of ‘correct’ capitalization.

19. Normalization to terms • One way is using Equivalence Classes: • car = automobile color = colour • Searches for one term will retrieve documents that contain each of these members. • We most commonly implicitly define equivalence classes of terms rather than being fully calculated in advance (hand constructed), e.g., • deleting periods to form a term • I.R.,IR • deleting hyphens to form a term • anti-discriminatory, antidiscriminatory

20. Sec. 2.2.3 Normalization: other languages • Accents: e.g., French résumé vs. resume. • Umlauts: e.g., German: Tuebingen vs. Tübingen • Normalization of things like date forms • 7月30日 vs. 7/30 • Tokenization and normalization may depend on the language and so is intertwined with language detection • Crucial: Need to “normalize” indexed text as well as query terms into the same form

21. Sec. 2.2.3 Normalization to terms • What is the disadvantage of equivalence classing? • An example: • Enter: window Search: window, windows • Enter: windows Search: Windows, windows, window • Enter: Windows Search: Windows • An alternative to equivalence classing is to do asymmetric expansion • It is hand constructed • Potentially more powerful, but less efficient

22. Stemming and lemmatization • Documents are going to use different forms of a word, • organize, organizes, and organizing • am, are,and is • There are families of derivationally related words with similar meanings, • democracy, democratic, and democratization. • Reduce tokens to their “roots” before indexing.

23. Sec. 2.2.4 Stemming • “Stemming” suggest crude affix chopping • language dependent • Example: • Porter’s algorithm • http://www.tartarus.org/~martin/PorterStemmer • Lovins stemmer • http://www.comp.lancs.ac.uk/computing/research/stemming/general/lovins.htm

24. Sec. 2.2.4 Typical rules in Porter • sses ss presses  press • ies i bodies  bodi • ss  ss press  press • s  cats  cat

25. Sec. 2.2.4 Lemmatization • Reduce inflectional/variant forms to base form (lemma) properly with the use of a vocabulary and morphological analysis of words • Lemmatizer: a tool from Natural Language Processing which does full morphological analysis to accurately identify the lemma for each word.

26. Sec. 2.2.4 Language-specificity • Many of the above features embody transformations that are • Language-specific and often, application-specific • There are “plug-in” addenda to the indexing process • Both open source and commercial plug-ins are available for handling these

27. Helpfulness of normalization • Do stemming and other normalizations help? • Definitely useful for Spanish, German, Finnish, … • 30% performance gains for Finnish! • What about English? • Not so considerable help! • Helps a lot for some queries, hurts performance a lot for others.

28. Helpfulness of normalization • Example: • operative (dentistry) ⇒ oper • operational (research) ⇒ oper • operating (systems) ⇒ oper • For a case like this, moving to using a lemmatizer would not completely fix the problem

29. Faster postings merges:Skip lists

30. 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!

31. Sec. 2.3 128 17 2 4 8 41 48 64 1 2 3 8 11 21 Augment postings with skip pointers • At indexing time. • The resulted list is skip list. 128 41 31 11 31

32. Sec. 2.3 17 2 4 8 41 48 64 1 2 3 8 11 21 But the skip successor of 11is 31, so we can skip ahead past the intervening postings. Query processing with skip pointers 128 41 128 31 11 31 Suppose we’ve stepped through the lists until we process 8 on each list. We then have 41 and 11. 11 is smaller.

33. Sec. 2.3 Where do we place skips? • Tradeoff: • More skips  More likely to skip. But lots of comparisons to skip pointers. • Fewer skips  Few successful skips. But few pointer comparison.

34. Sec. 2.3 Placing skips • Simple heuristic: for postings of length L, use L evenly-spaced skip pointers. • This ignores the distribution of query terms. • Easy if the index is relatively static; harder if L keeps changing because of updates. • The I/O cost of loading a bigger postings list can outweigh the gains from quicker in memory merging! D. Bahle, H. Williams, and J. Zobel. Efficient phrase querying with an auxiliary index. SIGIR 2002, pp. 215-221.

35. Phrase queries and positional indexes

36. Sec. 2.4 Phrase queries • 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. • Most recent search engines support a double quotes syntax

37. Phrase queries • PHRASE QUERIES has proven to be very easily understood and successfully used by users. • As many as 10% of web queries are phrase queries. • For this, it no longer suffices to store only <term : docs> entries • Solutions?

38. Sec. 2.4.1 A first attempt: Biword indexes • Index every consecutive pair of terms in the text as a phrase • For example the text “Qom computer department” would generate the biwords • Qom computer • computer department • Two-word phrase query-processing is now immediate.

39. Longer phrase queries • The query “modern information retrieval course” can be broken into the Boolean query on biwords: • modern information AND information retrieval AND retrieval course • Work fairly well in practice. • But there can and will be occasional errors.

40. Sec. 2.4.1 Issues for biword indexes • Errors, 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 but can be part of a compound strategy.

41. Sec. 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.>

42. Sec. 2.4.2 Positional index example • For phrase queries, we need to deal with more than just equality. <be: 993427; 1: 7, 18, 33, 72, 86, 231; 2: 3, 149; 4: 17, 191, 291, 430, 434; 5: 363, 367, …> Which of docs 1,2,4,5 could contain “to be or not to be”?

43. Sec. 2.4.2 Proximity queries • LIMIT /3 STATUTE /3 FEDERAL /2 TORT • /k means “within k words of (on either side)”. • Clearly, positional indexes can be used for such queries; biword indexes cannot.

45. Sec. 2.4.2 Postings Positional postings 1000 1 1 100,000 1 100 Document size Positional index size • Need an entry for each occurrence, not just once per document • Consider a term with frequency 0.1%

46. Sec. 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 “English-like” languages

47. Sec. 2.4.3 Combination schemes • These two approaches can be profitably combined • For particular phrases (“Hossein Rezazadeh”) it is inefficient to keep on merging positional postings lists

48. Combination schemes • 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 • H.E. Williams, J. Zobel, and D. Bahle. 2004. “Fast Phrase Querying with Combined Indexes”, ACM Transactions on Information Systems.

49. Exercise • Write a pseudo-code for biwork phrase queries using positional index. • Do exercises 2.5 and 2.6 of your book.