CS 430 / INFO 430 Information Retrieval Lecture 5 Searching Full Text 5 1 Course Administration 2 CS 430 / INFO 430 Information Retrieval Completion of Lecture 4 3 Word List
On disk If a word list is held on disk, search time is dominated by the number of disk accesses. In memory Suppose that a word list has 1,000,000 distinct terms. Each index entry consists of the term, some basic statistics and a pointer to the inverted list, average 100 characters. Size of index is 100 megabytes, which can easily be held in memory of a dedicated computer. 4 File Structures for Inverted Files: Linear Index Advantages Can be searched quickly, e.g., by binary search, O(log n) Good for lexicographic processing, e.g., comp* Convenient for batch updating Economical use of storage Disadvantages Index must be rebuilt if an extra term is added
5 File Structures for Inverted Files: Binary Tree Input: elk, hog, bee, fox, cat, gnu, ant, dog elk bee ant hog cat fox dog 6 gnu File Structures for Inverted Files: Binary Tree
Advantages Can be searched quickly Convenient for batch updating Easy to add an extra term Economical use of storage Disadvantages Less good for lexicographic processing, e.g., comp* Tree tends to become unbalanced If the index is held on disk, important to optimize the number of disk accesses 7 File Structures for Inverted Files: Binary Tree Calculation of maximum depth of tree. Worst case: depth = n O(n) Ideal case: depth = log(n + 1)/log 2 O(log n) Illustrates importance of balanced trees.
8 File Structures for Inverted Files: Right Threaded Binary Tree Threaded tree: A binary search tree in which each node uses an otherwise-empty left child link to refer to the node's inorder predecessor and an empty right child link to refer to its in-order successor. Right-threaded tree: A variant of a threaded tree in which only the right thread, i.e. link to the successor, of each node is maintained. Can be used for lexicographic processing. A good data structure when held in memory Knuth vol 1, 2.3.1, page 325. 9 File Structures for Inverted Files: Right Threaded Binary Tree dog gnu
bee ant cat hog elk fox 10 NULL File Structures for Inverted Files: B-trees B-tree of order m: A balanced, multiway search tree: Each node stores many keys
Root has between 2 and 2m keys. All other internal nodes have between m and 2m keys. If ki is the ith key in a given internal node -> all keys in the (i-1)th child are smaller than ki -> all keys in the ith child are bigger than ki All leaves are at the same depth 11 File Structures for Inverted Files: B-trees B-tree example (order 2) 50 65 55 59 10 19 35 36 47 1 5 8 9 12 14 18
12 21 24 28 70 90 98 66 68 91 95 97 72 73 Every arrow points to a node containing between 2 and 4 keys. A node with k keys has k + 1 pointers. File Structures for Inverted Files: B+-tree Example: B+-tree of order 2, bucket size 4 A B-tree is used as an index Data is stored in the leaves of the tree, known as buckets 50 65 10 25
... D9 55 59 D51 ... D54 70 81 90 D66... (Implementation of B+-trees is covered in CS 432.) 13 D81 ... CS 430 / INFO 430 Information Retrieval Lecture 5 Searching Full Text 5
14 SMART System An experimental system for automatic information retrieval automatic indexing to assign terms to documents and queries collect related documents into common subject classes identify documents to be retrieved by calculating similarities between documents and queries procedures for producing an improved search query
based on information obtained from earlier searches Gerald Salton and colleagues Harvard 1964-1968 Cornell 1968-1988 15 Indexing Subsystem Documents text assign document IDs break into tokens tokens *Indicates optional operation. documents
stop list* non-stoplist stemming* tokens stemmed terms term weighting* terms with weights 16 document numbers and *field numbers Index database
Boolean retrieved operations* document set relevant document set stemmed terms Index database Decisions in Building the Word List: What is a Term? 18 Underlying character set, e.g., printable ASCII, Unicode, UTF8.
Is there a controlled vocabulary? If so, what words are included? List of stopwords. Rules to decide the beginning and end of words, e.g., spaces or punctuation. Character sequences not to be indexed, e.g., sequences of numbers. Lexical Analysis: Term
What is a term? Free text indexing A term is a group of characters, extracted from the input string, that has some collective significance, e.g., a complete word. Usually, terms are strings of letters, digits or other specified characters, separated by punctuation, spaces, etc. 19 Oxford English Dictionary 20 Lexical Analysis: Choices Punctuation: In technical contexts, punctuation may be used as a character within a term, e.g., wordlist.txt. Case: Case of letters is usually not significant. Hyphens: (a) Treat as separators: state-of-art is treated as state of art.
(b) Ignore: on-line is treated as online. (c) Retain: Knuth-Morris-Pratt Algorithm is unchanged. Digits: Most numbers do not make good terms, but some are parts of proper nouns or technical terms: CS430, Opus 22. 21 Lexical Analysis: Choices The modern tendency, for free text searching, is to map upper and lower case letters together in index terms, but otherwise to minimize the changes made at the lexical analysis stage. 22 Lexical Analysis Example: Query Analyzer A term is a letter followed by a sequence of letters and digits. Upper case letters are mapped into the lower case equivalents. The following characters have significance as operators: (
23 ) & | Lexical Analysis: Transition Diagram letter, digit 1 space letter ( 2 3
) & 0 | other end-of-string 24 4 7 5 6 Lexical Analysis: Transition Table State space letter
0 1 0 1 1 1 ( ) & | other end-of string digit 2
1 3 1 4 1 5 1 6 1 States in red are final states. 25 6 1
7 1 Changing the Lexical Analyzer This use of a transition table allows the system administrator to establish differ lexical choices for different collections of documents. Example: To change the lexical analyzer to accept tokens that begin with a digit, change the top right element of the table to 1. 26 Stop Lists Very common words, such as of, and, the, are rarely of use in information retrieval. A stop list is a list of such words that are removed during lexical analysis. A long stop list saves space in indexes, speeds
processing, and eliminates many false hits. However, common words are sometimes significant in information retrieval, which is an argument for a short stop list. (Consider the query, "To be or not to be?") 27 Suggestions for Including Words in a Stop List Include the most common words in the English language (perhaps 50 to 250 words). Do not include words that might be important for retrieval (Among the 200 most frequently occurring words in general literature in English are time, war, home, life, water, and world). In addition, include words that are very common in context (e.g., computer, information, system in a set of computing documents). 28
Example: Stop List for Assignment 1 a are but has in more one that this which 29 about as by have is new or the
to will an at for he it of said their was with and be from his its on say
they who you Example: the WAIS stop list (first 84 of 363 multi-letter words) 30 about above according after afterwards again alone along already among amongst an anyone anything anywhere at be
becomes becoming been beginning behind being between beyond can can't could couldn't did didn't do down during each either else elsewhere across against also
another are became before below billion cannot actually adj all almost although always any anyhow aren't around because become beforehand begin beside
besides both but by caption co does eg end doesn't eight ending don't eighty enough Stop list policies How many words should be in the stop list? Long list lowers recall
Which words should be in list? Some common words may have retrieval importance: -- home, life, water, war, world In certain domains, some words are very common: -- computer, program, source, machine, language There is very little systematic evidence to use in selecting a stop list. 31 Stop Lists in Practice The modern tendency is: (a) have very short stop lists for broad-ranging or multi-lingual document collections, especially when the users are not trained. (b) have longer stop lists for document collections in well-defined fields, especially when the users are trained professional. 32 Stemming
Morphological variants of a word (morphemes). Similar terms derived from a common stem: engineer, engineered, engineering use, user, users, used, using Stemming in Information Retrieval. Grouping words with a common stem together. For example, a search on reads, also finds read, reading, and readable Stemming consists of removing suffixes and conflating the resulting morphemes. Occasionally, prefixes are also removed. 33 Categories of Stemmer The following diagram illustrate the various categories of stemmer. Porter's algorithm is shown by the red path. Conflation methods Manual Automatic (stemmers) Affix
removal Longest match 34 Successor variety Simple removal Table lookup n-gram Porter Stemmer A multi-step, longest-match stemmer. M. F. Porter, An algorithm for suffix stripping. (Originally
published in Program, 14 no. 3, pp 130-137, July 1980.) http:// www.tartarus.org/~martin/PorterStemmer/def.txt Notation v c (vc)m vowel constant (vowel followed by a constant) repeated m times Any word can be written: [c](vc)m[v] 35 m is called the measure of the word Porter's Stemmer Porter Stemming Algorithm Complex suffixes Complex suffixes are removed bit by bit in the different steps. Thus:
(*v*) ed null plastered -> plaster bled -> bled (*v*) ing null motoring -> motor sing -> sing *v* - the stem contains a vowel 38
Porter Stemmer: Step 5a Step 5a is defined as follows. (m>1) E -> (m=1 and not *o) E -> probate -> probat rate -> rate cease -> ceas *o - the stem ends cvc, where the second c is not W, X or Y (e.g. -WIL, -HOP). 39 Stemming in Practice
Evaluation studies have found that stemming can affect retrieval performance, usually for the better, but the results are mixed. Effectiveness is dependent on the vocabulary. Fine distinctions may be lost through stemming. Automatic stemming is as effective as manual conflation. Performance of various algorithms is similar. Porter's Algorithm is entirely empirical, but has proved to be an effective algorithm for stemming English text with trained users. 40 Selection of tokens, weights, stop lists and stemming Special purpose collections (e.g., law, medicine, monographs) Best results are obtained by tuning the search engine for the characteristics of the collections and the expected queries. It is valuable to use a training set of queries, with lists of relevant documents, to tune the system for each application. General purpose collections (e.g., news articles) The modern practice is to use a basic weighting scheme (e.g., tf.idf), a simple definition of token, a short stop list and little
stemming except for plurals, with minimal conflation. 41 Web searching combine similarity ranking with ranking based on document importance.
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