Lecture2 Dictionary

Penelusuran Informasi(Information Retrieval)

Sumber:CS276: Information Retrieval and Web Search

Pandu Nayak and Prabhakar Raghavan

Term Vocabulary & Postings lists(Tokenisasi)

Penelusuran Informasi (Information Retrieval)Penelusuran Informasi (Information Retrieval)

Pertemuan sebelumnya: Struktur dari Inverted Indeks:

Dictionary (Vocabulary) & Inverted List (Postings)

Vocabulary urut berdasarkan term (kata) Untuk memproses Boolean Query:

Melakukan interseksi (merging) secara linear

Ch. 1

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Topik Pada Pertemuan IniTahapan dalam Membangun Indeks Preprocessing untuk membentuk vocabulary

Documen Tokenisasi (tokenization) Kata (terms) apa saja yang dimasukkan dalam

indeks Inverted List (Postings)

Cara merge secara lebih cepat (faster merge) dengan cara skip lists

Query dalam bentuk kaliman (phrase)3

Introduction to Information RetrievalIntroduction to Information Retrieval

Diagram Proses Indexing

Tokenizer

Token stream Friends Romans Countrymen

Linguistic module

Modified tokens friend roman countryman

Indexer

Inverted index

friend

roman

countryman

2 4

2

13 16

1

Dokumen Friends, Romans, countrymen.

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Parsing Dokumen Perhatikan terlebih dahulu format dokumen

pdf/word/excel/html ? Ditulis dalam bahasa apa? Format character set yang digunakanBagaimana menentukan jawaban dari pertanyaan di atas?

Observasi secara manual? Atau dilakukan secara otomatis menggunakan metode klasifikasi?

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Complications: Format/Language Dokumen yang akan diindeks dapat berupa dokumen yang

ditulis dalam beberapa bahasa Sebuah indeks dapat mengandung kata dari beberapa bahasa Karena sebuah dokumen dapat ditulis dalam beberapa bahasa Contoh: Email dalam bahasa Inggris tetapi attacment dari email adalah

dokumen yang ditulis dalam bahasa Jerman Apakah unit dari sebuah dokumen?

Sebuah file? Sebuah email? Sebuah email dengan 5 attachments? Sekumpulan files (PPT atau halaman HTML)?

Sec. 2.1

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Introduction to Information RetrievalIntroduction to Information Retrieval

TOKENS & TERMS (KATA)

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Tokenisasi (Tokenization) Input: “Friends, Romans, Countrymen” Output: Tokens

Friends Romans Countrymen

Jadi token adalah sederetan karakter (a sequence of characters) dalam dokumen

Setiap token menjadi kandidat dari elemen dalam indeks, tentunya setelah preprocessing

Sec. 2.2.1

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Beberapa isu dalam tokenisasi: Finland’s capital Finland? Finlands? Finland’s? Hewlett-Packard Hewlett dan Packard sebagai

dua token atau satu? state-of-the-art: break up hyphenated sequence co-education lowercase, lower-case, lower case?

San Francisco: satu token atau dua? Bagaimana cara memutuskan bahwa SF adalah satu token?

Sec. 2.2.1

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Tokenisasi (Tokenization)


Angka (Numbers) 3/12/91 Mar. 12, 1991 12/3/91 No. B-52 Kode: 324a3df234cb23e Telepon: (0651) 234-2333

Biasanya angka memiliki space diantaranya Sistem IR yang lama tidak mengindeks angka

Tapi angka itu penting. Coba bayangkan bila ingin mencari baris dari error kode program melalui Sistem IR atau mencari nomor tertentu

Salah satu solusi adalah menggunakan mekanisme n-grams

Sec. 2.2.1

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Tokenisasi: Isu dalam bahasa French

L'ensemble satu token atau dua? L ? L’ ? Le ? Want l’ensemble to match with un ensemble

Sampai tahun 2003, tidak berhasil bila dicari via 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

Sec. 2.2.1

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Chinese and Japanese: 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: 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

Sec. 2.2.1

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Tokenisasi: Isu dalam bahasa


Tulisan Arab ditulis dari kanan ke kiri tetapi untuk angka dibaca dari kiri ke kanan

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.’

Sec. 2.2.1

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Tokenisasi: Isu dalam bahasa


Stop words

Menggunakan stop list, kata-kata yang sering muncul (tetapi kurang penting) dapat dikeluarkan dari indeks: Secara semantic mereka tidak penting: the, a, and, to, be Jumlahnya cukup banyak: ~30% dari semua kata dalam corpus

Trend: stopword tidak diikutkan: Hemat indeks dan dapat memperkecil ukuran indeks walaupun

dikompres Query optimisasi menjadi lebih baik Tapi perlu juga memperhatikan Query sbb:

Judul film: “King of Denmark” Judul Lagu: “Let it be”, “To be or not to be” Relational query: “flights to London”

Sec. 2.2.2

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Normalisasi Kata (terms) Kata harus dinormalisasidalam 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

Sec. 2.2.3

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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 de-accented term

Tuebingen, Tübingen, Tubingen Tubingen

Sec. 2.2.3

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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

Crucial: Need to “normalize” indexed text as well as query terms into the same form

Morgen will ich in MIT … Is this

German “mit”?

Sec. 2.2.3

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Case folding Reduce all letters to lower case

exception: upper case in mid-sentence? 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…

Google example: Query C.A.T. #1 result was for “cat” (well, Lolcats) not

Caterpillar Inc.

Sec. 2.2.3

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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

Sec. 2.2.3

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Thesauri and soundex Do we handle synonyms and homonyms?

E.g., by hand-constructed equivalence classes car = automobile color = colour

We can rewrite to form equivalence-class terms When the document contains automobile, index it under car-

automobile (and vice-versa) 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 lectures 3 and 9

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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

Sec. 2.2.4

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Stemming Reduce terms to their “roots” before indexing “Stemming” suggest crude affix chopping

language dependent e.g., automate(s), automatic, automation all reduced to

automat.

for example compressed and compression are both accepted as equivalent to compress.

for exampl compress andcompress ar both acceptas equival to compress

Sec. 2.2.4

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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.

Sec. 2.2.4

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Typical rules in Porter sses ss ies i ational ate tional tion

Rules sensitive to the measure of words (m>1) EMENT →

replacement → replac cement → cement

Sec. 2.2.4

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Other stemmers Other stemmers exist, e.g., Lovins stemmer

http://www.comp.lancs.ac.uk/computing/research/stemming/general/lovins.htm

Single-pass, longest suffix removal (about 250 rules)

Full morphological analysis – at most modest benefits for retrieval

Do stemming and other normalizations help? English: very mixed results. Helps recall but harms precision

operative (dentistry) ⇒ oper operational (research) ⇒ oper operating (systems) ⇒ oper

Definitely useful for Spanish, German, Finnish, … 30% performance gains for Finnish!

Sec. 2.2.4

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Recall basic merge Walk through the two postings simultaneously, in

time linear in the total number of postings entries

128

31

2 4 8 41 48 64

1 2 3 8 11 17 21

Brutus

Caesar2 8

If the list lengths are m and n, the merge takes O(m+n)operations.

Can we do better?Yes (if index isn’t changing too fast).

Sec. 2.3

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