Term Vocabulary and Postings Lists Vocabulary of Terms Tokenization language issues rench L'ensemble-> one token or two? LL Le Want 'ensemble to match with un ensemble Until at least 2003, it didnt on google Internationalization German noun compounds are not segmented a Lebensversicherungsgesellschaftsangestellter life insurance company employee German retrieval systems benefit greatly from a compound splitter module Can give a 15% performance boost for German
Term Vocabulary and Postings Lists 11 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 Vocabulary of Terms
Term Vocabulary and Postings Lists Vocabulary of Terms Tokenization language issues Chinese and japanese have no spaces between woras ■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 -元500社(情辍不足在的時刷方5005000万 Katakana Hiragana Kanji romaji End-user can express query entirely in hiragana
Term Vocabulary and Postings Lists 12 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 End-user can express query entirely in hiragana! Vocabulary of Terms
Term Vocabulary and Postings Lists Vocabulary of Terms 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 132-19624 usui sl Satul →<→> Ale gera achieved its independence in 1962 after 132 years of French occupation With Unicode the surface presentation is complex but the stored form is straightforward
Term Vocabulary and Postings Lists 13 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 ▪ ← → ← → ← ▪ ‘Algeria achieved its independence in 1962 after 132 years of French occupation.’ ▪ With Unicode, the surface presentationis complex, but the stored form is straightforward Vocabulary of Terms
Term Vocabulary and Postings Lists Vocabulary of Terms 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 (lecture 6)means the space for including stop words in a system is very small Good query optimization techniques(lecture 8 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
Term Vocabulary and Postings Lists 14 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 (lecture 6) means the space for including stop words in a system is very small ▪ Good query optimization techniques (lecture 8) 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” Vocabulary of Terms
Term Vocabulary and Postings Lists Vocabulary of Terms normalization to terms We 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 anti-discriminatory, antidiscriminatory antidiscriminatory
Term Vocabulary and Postings Lists 15 Normalization to terms ▪ We 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 ▪ anti-discriminatory, antidiscriminatory antidiscriminatory Vocabulary of Terms