CS 430: Information Discovery Lecture 2 Introduction to Text Based Information Retrieval

Course Administration • Campus store has run out of text books. More are on order. Reading for next week will be changed to not require the text book. • New Teaching Assistant, Yukiko Yamashita • Please send all questions about the course to: wya@cs.cornell.edu kyotov@cs.cornell.edu yukiko@cs.cornell.edu

Classical Information Retrieval media type text image, video, audio, etc. linking searching browsing CS 502 natural language processing catalogs, indexes (metadata) user-in-loop statistical CS 474

Documents A textual document is a digital object consisting of a sequence of words and other symbols, e.g., punctuation. The individual words and other symbols are known as tokens. A textual document can be: • Free text, also known as unstructured text, which is a continuous sequence of tokens. • Fielded text, also known as structured text, in which the text is broken into sections that are distinguished by tags or other markup. [Methods of markup, e.g., XML, are covered in CS 502.]

Word Frequency Observation: Some words are more common than others. Statistics: Most large collections of text documents have similar statistical characteristics. These statistics: • influence the effectiveness and efficiency of data structures used to index documents • many retrieval models rely on them The following example is taken from: Jamie Callan, Characteristics of Text, 1997 http://hobart.cs.umass.edu/~allan/cs646-f97/char_of_text.html

Rank Frequency Distribution For all the words in a collection of documents, for each word w f(w) is the frequency that w appears r(w) is rank of w in order of frequency, e.g., the most commonly occurring word has rank 1 f w has rank r and frequency f r

f f f the 1130021 from 96900 or 54958 of 547311 he 94585 about 53713 to 516635 million 3515 market 52110 a 464736 year 90104 they 51359 in 390819 its 86774 this 50933 and 387703 be 85588 would 50828 that 204351 was 83398 you 49281 for 199340 company 3070 which 48273 is 152483 an 76974 bank 47940 said 148302 has 74405 stock 47401 it 134323 are 74097 trade 47310 on 121173 have 73132 his 47116 by 118863 but 71887 more 46244 as 109135 will 71494 who 42142 at 101779 say 66807 one 41635 mr 101679 new 64456 their 40910 with 101210 share 63925

Zipf's Law If the words, w, in a collection are ranked, r(w), by their frequency, f(w), they roughly fit the relation: r(w) * f(w) = c Different collections have different constants c. In English text, c tends to be about n / 10, where n is the number of words in the collection. For a weird but wonderful discussion of this and many other examples of naturally occurring rank frequency distributions, see: Zipf, G. K., Human Behaviour and the Principle of Least Effort. Adison-Wesley, 1949

1000*rf/n 1000*rf/n 1000*rf/n the 59 from 92 or 101 of 58 he 95 about 102 to 82 million 98 market 101 a 98 year 100 they 103 in 103 its 100 this 105 and 122 be 104 would 107 that 75 was 105 you 106 for 84 company 109 which 107 is 72 an 105 bank 109 said 78 has 106 stock 110 it 78 are 109 trade 112 on 77 have 112 his 114 by 81 but 114 more 114 as 80 will 117 who 106 at 80 say 113 one 107 mr 86 new 112 their 108 with 91 share 114

Methods that Build on Zipf's Law Term weighting: Give differing weights to terms based on their frequency, with most frequent words weighed less. Stop lists: Ignore the most frequent words (upper cut-off) Significant words: Ignore the most frequent and least frequent words (upper and lower cut-off)

Luhn's Proposal "It is here proposed that the frequency of word occurrence in an article furnishes a useful measurement of word significance. It is further proposed that the relative position within a sentence of words having given values of significance furnish a useful measurement for determining the significance of sentences. The significance factor of a sentence will therefore be based on a combination of these two measurements." Luhn, H.P., The automatic creation of literature abstracts, IBM Journal of Research and Development, 2, 159-165 (1958)

Cut-off Levels for Significance Words Upper cut-off Lower cut-off Resolving power of significant words Significant words r from: Van Rijsbergen, Ch. 2

Information Retrieval Overview Similar Requests Documents Similar: mechanism for determining which information items meet the requirements of a given request.

Functional View of Information Retrieval Similar: mechanism for determining the similarity of the request representation to the information item representation. Documents Requests Index database

Major Subsystems Indexing subsystem: Receives incoming documents, converts them to the form required for the index and adds them to the index database. Search subsystem: Receives incoming requests, converts them to the form required for searching the index and searches the database for matching documents. The index database is the central hub of the system.

Example: Indexing Subsystem for Boolean Searching documents Documents assign document IDs text document numbers and *field numbers break into words words stoplist non-stoplist words stemming* *Indicates optional operation. stemmed words term weighting* terms with weights Index database from Frakes, page 7

Example: Search Subsystem for Boolean Searching query parse query query terms ranked document set stoplist non-stoplist words ranking* stemming* stemmed words relevance judgments* Boolean operations retrieved document set Index database *Indicates optional operation. relevant document set