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Statistical Learning for Text Classification
         with scikit-learn and NLTK

                Olivier Grisel
         http://twitter.com/ogrisel

              PyCon – 2011
Outline
●   Why text classification?
●   What is text classification?
●   How?
    ●   scikit-learn
    ●   NLTK
    ●   Google Prediction API
●   Some results
Applications of Text Classification
             Task                     Predicted outcome

Spam filtering                  Spam, Ham, Priority

Language guessing               English, Spanish, French, ...

Sentiment Analysis for Product
                               Positive, Neutral, Negative
Reviews
News Feed Topic                 Politics, Business, Technology,
Categorization                  Sports, ...
Pay-per-click optimal ads
                                Will yield clicks / Won't
placement
Recommender systems             Will I buy this book? / I won't
Supervised Learning Overview
●   Convert training data to a set of vectors of features
●   Build a model based on the statistical properties of
    features in the training set, e.g.
    ●   Naïve Bayesian Classifier
    ●   Logistic Regression
    ●   Support Vector Machines
●   For each new text document to classify
    ●   Extract features
    ●   Asked model to predict the most likely outcome
Summary
    Training          features
      Text            vectors
   Documents,
    Images,
    Sounds...
                                 Machine
                                 Learning
                                 Algorithm
     Labels




  New
  Text          features
Document,       vector           Predictive   Expected
 Image,                            Model       Label
 Sound
Bags of Words
●   Tokenize document: list of uni-grams
      ['the', 'quick', 'brown', 'fox', 'jumps', 'over', 'the', 'lazy', 'dog']
●   Binary occurrences / counts:
      {'the': True, 'quick': True...}
●   Frequencies:
       {'the': 0.22, 'quick': 0.11, 'brown': 0.11, 'fox': 0.11…}
●   TF-IDF
      {'the': 0.001, 'quick': 0.05, 'brown': 0.06, 'fox': 0.24…}
Better than frequencies: TF-IDF
●   Term Frequency



●   Inverse Document Frequency




●   Non informative words such as “the” are scaled done
Even better features
●   bi-grams of words:
    ●   “New York”, “very bad”, “not good”
●   n-grams of chars:
    ●   “the”, “ed ”, “ a ” (useful for language guessing)
●   Combine with:
    ●   Binary occurrences
    ●   Frequencies
    ●   TF-IDF
scikit-learn
scikit-learn
●   BSD
●   numpy / scipy / cython / c++ wrappers
●   Many state of the art implementations
●   A new release every 3 months
●   17 contributors on release 0.7
●   Not just for text classification
Features Extraction in scikit-learn
from scikits.learn.features.text import WordNGramAnalyzer
text = (u"J'ai mangxe9 du kangourou ce midi,"
       u" c'xe9tait pas trxeas bon.")


WordNGramAnalyzer(min_n=1, max_n=2).analyze(text)
[u'ai', u'mange', u'du', u'kangourou', u'ce', u'midi', u'etait',
u'pas', u'tres', u'bon', u'ai mange', u'mange du', u'du
kangourou', u'kangourou ce', u'ce midi', u'midi etait', u'etait
pas', u'pas tres', u'tres bon']
Features Extraction in scikit-learn
from scikits.learn.features.text import CharNGramAnalyzer


analyzer = CharNGramAnalyzer(min_n=3, max_n=6)
char_ngrams = analyzer.analyze(text)


print char_ngrams[:5] + char_ngrams[-5:]
[u"j'a", u"'ai", u'ai ', u'i m', u' ma', u's tres', u' tres ', u'tres b',
u'res bo', u'es bon']
TF-IDF features & SVMs
from scikits.learn.features.text.sparse import Vectorizer
from scikits.learn.sparse.svm.sparse import LinearSVC



vec = Vectorizer(analyzer=analyzer)

features = vec.fit_transform(list_of_documents)

clf = LinearSVC(C=100).fit(features, labels)



clf2 = pickle.loads(pickle.dumps(clf))

predicted_labels = clf2.predict(features_of_new_docs)
)
                                    cs
                                (do
                        form
                      ns
                  .tra
    Training                 features
      Text       c
   Documents, ve             vectors
     Images,
     Sounds...
                                             )   Machine
                                          X,y
                                     fit(
                                                 Learning
                                   .
                               clf               Algorithm
      Labels

                                       w)
                                 _   ne                                                w)
                               cs                                                  _n
                                                                                      e
                           (do                                                   X
                         m                                                   ct(
                     sfor                                                ed
                                                                           i
  New             an                                              .   pr
  Text       c.t
                 r                                            clf
           e              features
Document, v               vector                 Predictive                     Expected
 Image,                                            Model                         Label
 Sound
NLTK
●   Code: ASL 2.0 & Book: CC-BY-NC-ND
●   Tokenizers, Stemmers, Parsers, Classifiers,
    Clusterers, Corpus Readers
NLTK Corpus Downloader
>>> import nltk
>>> nltk.download()
Using a NLTK corpus
>>> from nltk.corpus import movie_reviews as reviews


>>> pos_ids = reviews.fileids('pos')
>>> neg_ids = reviews.fileids('neg')
>>> len(pos_ids), len(neg_ids)
1000, 1000


>>> reviews.words(pos_ids[0])
['films', 'adapted', 'from', 'comic', 'books', 'have', ...]
Common data cleanup operations
●   Lower case & remove accentuated chars:
import unicodedata
s = ''.join(c for c in unicodedata.normalize('NFD', s.lower())
             if unicodedata.category(c) != 'Mn')
●   Extract only word tokens of at least 2 chars
    ●   Using NLTK tokenizers & stemmers
    ●   Using a simple regexp:
        re.compile(r"bww+b", re.U).findall(s)
Feature Extraction with NLTK
                   Unigram features




def word_features(words):
   return dict((word, True) for word in words)
Feature Extraction with NLTK
                     Bigram Collocations
from nltk.collocations import BigramCollocationFinder
from nltk.metrics import BigramAssocMeasures as BAM
from itertools import chain


def bigram_features(words, score_fn=BAM.chi_sq):
   bg_finder = BigramCollocationFinder.from_words(words)
   bigrams = bg_finder.nbest(score_fn, 100000)
   return dict((bg, True) for bg in chain(words, bigrams))
The NLTK Naïve Bayes Classifier
from nltk.classify import NaiveBayesClassifier


neg_examples = [(features(reviews.words(i)), 'neg') for i in neg_ids]
pos_examples = [(features(reviews.words(i)), 'pos') for i in pos_ids]
train_set = pos_examples + neg_examples


classifier = NaiveBayesClassifier.train(train_set)
Most informative features
>>> classifier.show_most_informative_features()
     magnificent = True         pos : neg          =     15.0 : 1.0
     outstanding = True         pos : neg          =      13.6 : 1.0
      insulting = True        neg : pos     =          13.0 : 1.0
      vulnerable = True        pos : neg           =     12.3 : 1.0
      ludicrous = True         neg : pos       =        11.8 : 1.0
        avoids = True         pos : neg        =       11.7 : 1.0
     uninvolving = True         neg : pos          =     11.7 : 1.0
      astounding = True         pos : neg          =      10.3 : 1.0
     fascination = True        pos : neg       =         10.3 : 1.0
       idiotic = True        neg : pos     =           9.8 : 1.0
Training NLTK classifiers
●   Try nltk-trainer

●   python train_classifier.py --instances paras 
    --classifier NaiveBayes –bigrams 
    --min_score 3      movie_reviews
REST services
NLTK – Online demos
NLTK – REST APIs
% curl -d "text=Inception is the best movie ever" 
            http://text-processing.com/api/sentiment/


{
    "probability": {
         "neg": 0.36647424288117808,
         "pos": 0.63352575711882186
    },
    "label": "pos"
}
Google Prediction API
Statistical Machine Learning for Text Classification with scikit-learn and NLTK
Statistical Machine Learning for Text Classification with scikit-learn and NLTK
Typical performance results:
                   movie reviews
●   nltk:
    ● unigram occurrences
    ● Naïve Bayesian Classifier          ~ 70%
●   Google Prediction API                ~ 83%
●   scikit-learn:
    ●  TF-IDF unigram features
    ● LinearSVC                          ~ 87%
●   nltk:
    ●   Collocation features selection
    ●   Naïve Bayesian Classifier        ~ 97%
Typical results:
    newsgroups topics classification

●   20 newsgroups dataset
    ●   ~ 19K short text documents
    ●   20 categories
    ●   By date train / test split

●   Bigram TF-IDF + LinearSVC:       ~ 87%
Confusion Matrix (20 newsgroups)
00 alt.atheism
01 comp.graphics
02 comp.os.ms-windows.misc
03 comp.sys.ibm.pc.hardware
04 comp.sys.mac.hardware
05 comp.windows.x
06 misc.forsale
07 rec.autos
08 rec.motorcycles
09 rec.sport.baseball
10 rec.sport.hockey
11 sci.crypt
12 sci.electronics
13 sci.med
14 sci.space
15 soc.religion.christian
16 talk.politics.guns
17 talk.politics.mideast
18 talk.politics.misc
19 talk.religion.misc
Typical results:
           Language Identification


●   15 Wikipedia articles
●   [p.text_content() for p in html_tree.findall('//p')]
●   CharNGramAnalyzer(min_n=1, max_n=3)
●   TF-IDF
●   LinearSVC
Typical results:
Language Identification
Scaling to many possible outcomes
●   Example: possible outcomes are all the
    categories of Wikipedia (565,108)
●   From Document Categorization
                           to Information Retrieval
●   Fulltext index for TF-IDF similarity queries
●   Smart way to find the top 30 search keywords
●   Use Apache Lucene / Solr MoreLikeThisQuery
Some pointers
●   http://scikit-learn.sf.net               doc & examples
    http://github.com/scikit-learn                     code
●   http://www.nltk.org              code & doc & PDF book
●   http://streamhacker.com/
    ●   Jacob Perkins' blog on NLTK & APIs
●   https://github.com/japerk/nltk-trainer
●   http://www.slideshare.net/ogrisel             these slides
●   http://twitter.com/ogrisel / http://github.com/ogrisel

                             Questions?

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Statistical Machine Learning for Text Classification with scikit-learn and NLTK

  • 1. Statistical Learning for Text Classification with scikit-learn and NLTK Olivier Grisel http://twitter.com/ogrisel PyCon – 2011
  • 2. Outline ● Why text classification? ● What is text classification? ● How? ● scikit-learn ● NLTK ● Google Prediction API ● Some results
  • 3. Applications of Text Classification Task Predicted outcome Spam filtering Spam, Ham, Priority Language guessing English, Spanish, French, ... Sentiment Analysis for Product Positive, Neutral, Negative Reviews News Feed Topic Politics, Business, Technology, Categorization Sports, ... Pay-per-click optimal ads Will yield clicks / Won't placement Recommender systems Will I buy this book? / I won't
  • 4. Supervised Learning Overview ● Convert training data to a set of vectors of features ● Build a model based on the statistical properties of features in the training set, e.g. ● Naïve Bayesian Classifier ● Logistic Regression ● Support Vector Machines ● For each new text document to classify ● Extract features ● Asked model to predict the most likely outcome
  • 5. Summary Training features Text vectors Documents, Images, Sounds... Machine Learning Algorithm Labels New Text features Document, vector Predictive Expected Image, Model Label Sound
  • 6. Bags of Words ● Tokenize document: list of uni-grams ['the', 'quick', 'brown', 'fox', 'jumps', 'over', 'the', 'lazy', 'dog'] ● Binary occurrences / counts: {'the': True, 'quick': True...} ● Frequencies: {'the': 0.22, 'quick': 0.11, 'brown': 0.11, 'fox': 0.11…} ● TF-IDF {'the': 0.001, 'quick': 0.05, 'brown': 0.06, 'fox': 0.24…}
  • 7. Better than frequencies: TF-IDF ● Term Frequency ● Inverse Document Frequency ● Non informative words such as “the” are scaled done
  • 8. Even better features ● bi-grams of words: ● “New York”, “very bad”, “not good” ● n-grams of chars: ● “the”, “ed ”, “ a ” (useful for language guessing) ● Combine with: ● Binary occurrences ● Frequencies ● TF-IDF
  • 10. scikit-learn ● BSD ● numpy / scipy / cython / c++ wrappers ● Many state of the art implementations ● A new release every 3 months ● 17 contributors on release 0.7 ● Not just for text classification
  • 11. Features Extraction in scikit-learn from scikits.learn.features.text import WordNGramAnalyzer text = (u"J'ai mangxe9 du kangourou ce midi," u" c'xe9tait pas trxeas bon.") WordNGramAnalyzer(min_n=1, max_n=2).analyze(text) [u'ai', u'mange', u'du', u'kangourou', u'ce', u'midi', u'etait', u'pas', u'tres', u'bon', u'ai mange', u'mange du', u'du kangourou', u'kangourou ce', u'ce midi', u'midi etait', u'etait pas', u'pas tres', u'tres bon']
  • 12. Features Extraction in scikit-learn from scikits.learn.features.text import CharNGramAnalyzer analyzer = CharNGramAnalyzer(min_n=3, max_n=6) char_ngrams = analyzer.analyze(text) print char_ngrams[:5] + char_ngrams[-5:] [u"j'a", u"'ai", u'ai ', u'i m', u' ma', u's tres', u' tres ', u'tres b', u'res bo', u'es bon']
  • 13. TF-IDF features & SVMs from scikits.learn.features.text.sparse import Vectorizer from scikits.learn.sparse.svm.sparse import LinearSVC vec = Vectorizer(analyzer=analyzer) features = vec.fit_transform(list_of_documents) clf = LinearSVC(C=100).fit(features, labels) clf2 = pickle.loads(pickle.dumps(clf)) predicted_labels = clf2.predict(features_of_new_docs)
  • 14. ) cs (do form ns .tra Training features Text c Documents, ve vectors Images, Sounds... ) Machine X,y fit( Learning . clf Algorithm Labels w) _ ne w) cs _n e (do X m ct( sfor ed i New an . pr Text c.t r clf e features Document, v vector Predictive Expected Image, Model Label Sound
  • 15. NLTK ● Code: ASL 2.0 & Book: CC-BY-NC-ND ● Tokenizers, Stemmers, Parsers, Classifiers, Clusterers, Corpus Readers
  • 16. NLTK Corpus Downloader >>> import nltk >>> nltk.download()
  • 17. Using a NLTK corpus >>> from nltk.corpus import movie_reviews as reviews >>> pos_ids = reviews.fileids('pos') >>> neg_ids = reviews.fileids('neg') >>> len(pos_ids), len(neg_ids) 1000, 1000 >>> reviews.words(pos_ids[0]) ['films', 'adapted', 'from', 'comic', 'books', 'have', ...]
  • 18. Common data cleanup operations ● Lower case & remove accentuated chars: import unicodedata s = ''.join(c for c in unicodedata.normalize('NFD', s.lower()) if unicodedata.category(c) != 'Mn') ● Extract only word tokens of at least 2 chars ● Using NLTK tokenizers & stemmers ● Using a simple regexp: re.compile(r"bww+b", re.U).findall(s)
  • 19. Feature Extraction with NLTK Unigram features def word_features(words): return dict((word, True) for word in words)
  • 20. Feature Extraction with NLTK Bigram Collocations from nltk.collocations import BigramCollocationFinder from nltk.metrics import BigramAssocMeasures as BAM from itertools import chain def bigram_features(words, score_fn=BAM.chi_sq): bg_finder = BigramCollocationFinder.from_words(words) bigrams = bg_finder.nbest(score_fn, 100000) return dict((bg, True) for bg in chain(words, bigrams))
  • 21. The NLTK Naïve Bayes Classifier from nltk.classify import NaiveBayesClassifier neg_examples = [(features(reviews.words(i)), 'neg') for i in neg_ids] pos_examples = [(features(reviews.words(i)), 'pos') for i in pos_ids] train_set = pos_examples + neg_examples classifier = NaiveBayesClassifier.train(train_set)
  • 22. Most informative features >>> classifier.show_most_informative_features() magnificent = True pos : neg = 15.0 : 1.0 outstanding = True pos : neg = 13.6 : 1.0 insulting = True neg : pos = 13.0 : 1.0 vulnerable = True pos : neg = 12.3 : 1.0 ludicrous = True neg : pos = 11.8 : 1.0 avoids = True pos : neg = 11.7 : 1.0 uninvolving = True neg : pos = 11.7 : 1.0 astounding = True pos : neg = 10.3 : 1.0 fascination = True pos : neg = 10.3 : 1.0 idiotic = True neg : pos = 9.8 : 1.0
  • 23. Training NLTK classifiers ● Try nltk-trainer ● python train_classifier.py --instances paras --classifier NaiveBayes –bigrams --min_score 3 movie_reviews
  • 26. NLTK – REST APIs % curl -d "text=Inception is the best movie ever" http://text-processing.com/api/sentiment/ { "probability": { "neg": 0.36647424288117808, "pos": 0.63352575711882186 }, "label": "pos" }
  • 30. Typical performance results: movie reviews ● nltk: ● unigram occurrences ● Naïve Bayesian Classifier ~ 70% ● Google Prediction API ~ 83% ● scikit-learn: ● TF-IDF unigram features ● LinearSVC ~ 87% ● nltk: ● Collocation features selection ● Naïve Bayesian Classifier ~ 97%
  • 31. Typical results: newsgroups topics classification ● 20 newsgroups dataset ● ~ 19K short text documents ● 20 categories ● By date train / test split ● Bigram TF-IDF + LinearSVC: ~ 87%
  • 32. Confusion Matrix (20 newsgroups) 00 alt.atheism 01 comp.graphics 02 comp.os.ms-windows.misc 03 comp.sys.ibm.pc.hardware 04 comp.sys.mac.hardware 05 comp.windows.x 06 misc.forsale 07 rec.autos 08 rec.motorcycles 09 rec.sport.baseball 10 rec.sport.hockey 11 sci.crypt 12 sci.electronics 13 sci.med 14 sci.space 15 soc.religion.christian 16 talk.politics.guns 17 talk.politics.mideast 18 talk.politics.misc 19 talk.religion.misc
  • 33. Typical results: Language Identification ● 15 Wikipedia articles ● [p.text_content() for p in html_tree.findall('//p')] ● CharNGramAnalyzer(min_n=1, max_n=3) ● TF-IDF ● LinearSVC
  • 35. Scaling to many possible outcomes ● Example: possible outcomes are all the categories of Wikipedia (565,108) ● From Document Categorization to Information Retrieval ● Fulltext index for TF-IDF similarity queries ● Smart way to find the top 30 search keywords ● Use Apache Lucene / Solr MoreLikeThisQuery
  • 36. Some pointers ● http://scikit-learn.sf.net doc & examples http://github.com/scikit-learn code ● http://www.nltk.org code & doc & PDF book ● http://streamhacker.com/ ● Jacob Perkins' blog on NLTK & APIs ● https://github.com/japerk/nltk-trainer ● http://www.slideshare.net/ogrisel these slides ● http://twitter.com/ogrisel / http://github.com/ogrisel Questions?