Introduction to text classification using naive bayes
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This document provides an overview of text classification and the Naive Bayes classification method. It defines text classification as assigning categories, topics or genres to documents. It describes classification methods like hand-coded rules and supervised machine learning. It explains the bag-of-words representation and how Naive Bayes classification works by calculating the probability of a document belonging to a class using Bayes' rule and independence assumptions. It discusses parameter estimation and how to build a multinomial Naive Bayes classifier for text classification tasks.
Introduction to text classification using naive bayes
- 2. Positive or negative movie review? • unbelievably disappointing • Full of zany characters and richly applied satire, and some great plot twists • this is the greatest screwball comedy ever filmed • It was pathetic. The worst part about it was the boxing scenes. 2
- 3. What is the subject of this article? • Management/mba • admission • arts • exam preparation • nursing • technology • … 3 Subject Category ?
- 4. Text Classification • Assigning subject categories, topics, or genres • Spam detection • Authorship identification • Age/gender identification • Language Identification • Sentiment analysis • …
- 5. Text Classification: definition • Input: • a document d • a fixed set of classes C = {c1, c2,…, cJ} • Output: a predicted class c ∈ C
- 6. Classification Methods: Hand-coded rules • Rules based on combinations of words or other features • spam: black-list-address OR (“dollars” AND“have been selected”) • Accuracy can be high • If rules carefully refined by expert • But building and maintaining these rules is expensive
- 7. Classification Methods: Supervised Machine Learning • Input: • a document d • a fixed set of classes C = {c1, c2,…, cJ} • A training set of m hand-labeled documents (d1,c1),....,(dm,cm) • Output: • a learned classifier γ:d c 7
- 8. Classification Methods: Supervised Machine Learning • Any kind of classifier • Naïve Bayes • Logistic regression • Support-vector machines • Maximum Entropy Model • Generative Vs Discriminative • …
- 9. Naïve Bayes Intuition • Simple (“naïve”) classification method based on Bayes rule • Relies on very simple representation of document • Bag of words
- 10. The bag of words representation I love this movie! It's sweet, but with satirical humor. The dialogue is great and the adventure scenes are fun… It manages to be whimsical and romantic while laughing at the conventions of the fairy tale genre. I would recommend it to just about anyone. I've seen it several times, and I'm always happy to see it again whenever I have a friend who hasn't seen it yet. γ ( )=c
- 11. The bag of words representation I love this movie! It's sweet, but with satirical humor. The dialogue is great and the adventure scenes are fun… It manages to be whimsical and romantic while laughing at the conventions of the fairy tale genre. I would recommend it to just about anyone. I've seen it several times, and I'm always happy to see it again whenever I have a friend who hasn't seen it yet. γ ( )=c
- 12. The bag of words representation: using a subset of words x love xxxxxxxxxxxxxxxx sweet xxxxxxx satirical xxxxxxxxxx xxxxxxxxxxx great xxxxxxx xxxxxxxxxxxxxxxxxxx fun xxxx xxxxxxxxxxxxx whimsical xxxx romantic xxxx laughing xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxx recommend xxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xx several xxxxxxxxxxxxxxxxx xxxxx happy xxxxxxxxx again xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxx γ ( )=c
- 13. Planning GUIGarbage Collection Machine Learning NLP parser tag training translation language... learning training algorithm shrinkage network... garbage collection memory optimization region... Test document parser language label translation … Bag of words for document classification ...planning temporal reasoning plan language... ?
- 14. Bayes’ Rule Applied to Documents and Classes • For a document d and a class c P(c| d) = P(d| c)P(c) P(d)
- 15. Naïve Bayes Classifier (I) MAP is “maximum a posteriori” = most likely class Bayes Rule Dropping the denominator cMAP = argmax c∈C P(c| d) = argmax c∈C P(d| c)P(c) P(d) = argmax c∈C P(d| c)P(c)
- 16. Naïve Bayes Classifier (II) Document d represented as features x1..xn cMAP = argmax c∈C P(d| c)P(c) = argmax c∈C P(x1, x2,…, xn | c)P(c)
- 17. Naïve Bayes Classifier (IV) How often does this class occur? O(|X|n•|C|) parameters We can just count the relative frequencies in a corpus Could only be estimated if a very, very large number of training examples was available. cMAP = argmax c∈C P(x1, x2,…, xn | c)P(c)
- 18. Multinomial Naïve Bayes Independence Assumptions • Bag of Words assumption: Assume position doesn’t matter • Conditional Independence: Assume the feature probabilities P(xi|cj) are independent given the class c. P(x1, x2,…, xn | c) P(x1,…, xn |c) = P(x1 |c)•P(x2 |c)•P(x3 |c)•...•P(xn | c)
- 19. Multinomial Naïve Bayes Classifier cMAP = argmax c∈C P(x1, x2,…, xn | c)P(c) cNB = argmax c∈C P(cj ) P(x| c) x∈X ∏
- 20. Learning the Multinomial Naïve Bayes Model • First attempt: maximum likelihood estimates • simply use the frequencies in the data ˆP(wi | cj ) = count(wi,cj ) count(w,cj ) w∈V ∑ ˆP(cj ) = doccount(C = cj ) Ndoc
- 21. • Create mega-document for topic j by concatenating all docs in this topic • Use frequency of w in mega-document Parameter estimation fraction of times word wi appears among all words in documents of topic cj ˆP(wi | cj ) = count(wi,cj ) count(w,cj ) w∈V ∑
- 22. 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
- 23. Real-world systems generally combine: • Automatic classification • Manual review of uncertain/difficult/"new” cases 23
- 24. 24 The Real World • Gee, I’m building a text classifier for real, now! • What should I do?
- 25. 25 The Real World • Write your own classifier code. • Tools: ● Apache Mahout (java) ● NLTK (python) ● Lingpipe ● Stanford Classifier ….. • APIs: ● OpenCalais ● AlchemiApi ● UIUC CCG.....