Text Classification and Naïve Bayes Multinomial Naïve Bayes: A Worked Example.

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Presentation transcript:

Text Classification and Naïve Bayes Multinomial Naïve Bayes: A Worked Example

Dan Jurafsky Choosing a class: P(c|d5) P(j|d5) 1/4 * (2/9) 3 * 2/9 * 2/9 ≈ DocWordsClass Training1Chinese Beijing Chinesec 2Chinese Chinese Shanghaic 3Chinese Macaoc 4Tokyo Japan Chinesej Test5Chinese Chinese Chinese Tokyo Japan? 2 Conditional Probabilities: P(Chinese|c) = P(Tokyo|c) = P(Japan|c) = P(Chinese|j) = P(Tokyo|j) = P(Japan|j) = Priors: P(c)= P(j)= (5+1) / (8+6) = 6/14 = 3/7 (0+1) / (8+6) = 1/14 (1+1) / (3+6) = 2/9 (0+1) / (8+6) = 1/14 (1+1) / (3+6) = 2/9 3/4 * (3/7) 3 * 1/14 * 1/14 ≈

Dan Jurafsky Naïve Bayes in Spam Filtering SpamAssassin Features: Mentions Generic Viagra Online Pharmacy Mentions millions of (dollar) ((dollar) NN,NNN,NNN.NN) Phrase: impress... girl From: starts with many numbers Subject is all capitals HTML has a low ratio of text to image area One hundred percent guaranteed Claims you can be removed from the list 'Prestigious Non-Accredited Universities'

Dan Jurafsky Summary: Naive Bayes is Not So Naive Very Fast, low storage requirements Robust to Irrelevant Features Irrelevant Features cancel each other without affecting results Very good in domains with many equally important features Decision Trees suffer from fragmentation in such cases – especially if little data Optimal if the independence assumptions hold: If assumed independence is correct, then it is the Bayes Optimal Classifier for problem A good dependable baseline for text classification But we will see other classifiers that give better accuracy

Text Classification and Naïve Bayes Multinomial Naïve Bayes: A Worked Example