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Words and Terms

Words and Terms. Stopwords and Stemmers. Last Weeks Key Points. The web is huge The web has a high rate of change The need to speed interactive searching by preparing an index in advance The need for lexical analysis and spam filtering. Worth Storing Every Word.

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Words and Terms

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  1. Words and Terms Stopwords and Stemmers

  2. Last Weeks Key Points • The web is huge • The web has a high rate of change • The need to speed interactive searching by preparing an index in advance • The need for lexical analysis and spam filtering

  3. Worth Storing Every Word • Is it worth recording every page containing “the” ?

  4. Stop Words • Words which are too frequent to differentiate between pages and are not worth storing in the index are called “stop words” • Good list at • http://www.dcs.gla.ac.uk/idom/ir_resources/linguistic_utils/stop_words

  5. PERL Example for Stopwords ….. Foreach $stopword (@stopwords) { $line=~ s/$stopword//g }

  6. Stemming • Reducing morphological variants of words to a standard underlying form • e.g. calculate, calculates, calculations to calculat- • improves recall at the expense of precision

  7. Simple PERL Stemmer $line=~ s/s / /g $line=~ s/ed / /g

  8. Porter Stemming Algorithm • Well known, effective stemmer, which does not use a dictionary • uses measure m • C(VC)mV • where • C is a sequence of consonants • V is a sequence of vowels

  9. -sses -ing -at -y -ss - -ate -i Porter Algorithm Step 1 Stem only vowels Stem only vowels

  10. -aliti -icate -able -al -ic - Porter Algorithm Step 2-4 Measure >0 Measure >0 Measure >1

  11. Official PERL Implementation http://www.tartarus.org/ ~martin/PorterStemmer/index.html

  12. Dictionary Based Stemmers • Dictionary of stems • cf vector based methods • Dictionary of words • effective handling of irregular forms • Proper Name/Controlled Vocabulary Lists • Equivalent Term/Thesaurii

  13. Problems with stemming • Always worsens precision hoping to improve recall • Causes (sometimes odd misretrieval) • “bled” vs “bleeding” • incorrect term conflation “plastered” to “plaster” • Do we really want to improve recall on the web ?

  14. Keypoints • Stop words • Why we need them • Porter Stemmer • Easy way to improve matches • Other Forms of Stemmer • Dictionaries and Thesauri

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