A Survey on Text Classification


1
A Survey on Text Classification

• December 10 2003
• 20033077 Dongho Kim
• KAIST

2
Contents

• Introduction
• Statistical Properties of Text
• Feature Selection
• Feature Space Reduction
• Classification Methods
• Using SVM and TSVM
• Hierarchical Text Classification
• Summary

3
Introduction

• Text classification
• Assign text to predefined categories based on
  content
• Types of text
• Documents (typical)
• Paragraphs
• Sentences
• WWW-Sites
• Different types of categories
• By topic
• By function
• By author
• By style

4
Text Classification Example
5
Computer-Based Text Classification Technologies

• Naive word-matching (Chute Yang Buntrock
  1994)
• Finding shared words between the text and names
  of categories
• Weakest method
• Cannot capture any conceptually relation
• Thesaurus-based matching (Lindberg Humphreys
  1990)
• Using lexical links
• Insensitive to the context
• High cost and low adaptivity across domains

6
Computer-Based Text Classification Technologies

• Empirical learning of term-category associations
• Learning from a training set
• Fundamentally different from word-matching
• Statistically capturing the semantic association
  between terms and categories
• Context sensitive mapping from terms to
  categories
• For example
• Decision tree methods
• Bayesian belief networks
• Neural networks
• Nearest neighbor classification methods
• Least-squares regression techiniques

7
Statistical Properties of Text

• There are stable language-independent patterns
  in how people use natural language
• A few words occur very frequently most occur
  rarely
• In general
• Top 2 words 1015 of all word occurrences
• Top 6 words 20 of all word occurrences
• Top 50 words 50 of all word occurrences

Most common words from Tom Sawyer
1 14
8
Statistical Properties of Text

• The most frequent words in one corpus may be rare
  words in another corpus
• Example computer in CACM vs. National
  Geographic
• Each corpus has a different fairly small
  working vocabulary

These properties hold in a wide range of languages
9
Statistical Properties of Text

• Summary
• Term usage is highly skewed but in a predictable
  pattern
• Why is it important to know the characteristics
  of text
• Optimization of data structures
• Statistical retrieval algorithms depend on them

10
Statistical Profiles

• Can act as a summarization device
• Indicate what a document is about
• Indicate what a collection is about

11
Zipfs Law

• Zipfs Law relates a terms frequency to its rank
• Frequency 1/rank
• There is a constant such that
• Rank the terms in a vocabulary by frequency in
  descending order
• Empirical observation
• Hence
• for English

12
Precision and Recall
Evaluation Metrics

• Recall
• Percentage of all relevant documents that are
  found by a search
• Precision
• Percentage of retrieved documents that are
  relevant

13
F-measure
Evaluation Metrics
Harmonic average of precision and recall

• Rewards results that keep recall and precision
  close together
• R40 P60. R/P average50. F-measure48
• R45 P55. R/P average50. F-measure49.5

14
Break Even Point
Evaluation Metrics

• The point at which recall equals precision

Evaluation metric The value of this point
15
Term Weights A Brief Introduction
Feature Selection

• The words of a text are not equally indicative of
  its meaning
• Important butterflies monarchs scientists
  direction compass
• Unimportant most think kind sky determine
  cues learn
• Term weights reflect the (estimated) importance
  of each term

Most scientists think that butterflies use the
position of the sun in the sky as a kind of
compass that allows them to determine which way
is north. Scientists think that butterflies may
use other cues such as the earths magnetic
field but we have a lot to learn about monarchs
sense of direction.
16
Term Weights
Feature Selection

• Term frequency (TF)
• The more often a word occurs in a document the
  better that term is in describing what the
  document is about
• Often normalized e.g. by the length of the
  document
• Sometimes biased to range 0.4..1.0 to represent
  the fact that even a single occurrence of a term
  is a significant event

17
Term Weights
Feature Selection

• Inverse document frequency (IDF)
• Terms that occur in many documents in the
  collection are less useful for discriminating
  among documents
• Document frequency (df) number of documents
  containing the term
• IDF often calculated as
• TF and IDF are used in combination as product

18
Vector Space Similarity
Feature Selection

• Similarity is inversely related to the angle
  between the vectors
• Cosine of the angle between the two vectors

19
Feature Space Reduction

• Main reasons
• Improve accuracy of the algorithm
• Decrease the size of data set
• Control the computation time
• Avoid overfitting
• Feature space reduction technique
• Stopword removal stemming
• Information gain
• Natural language processing

20
Stopword Removal
Feature Space Reduction

• Stopwords words that are discarded from a
  document representation
• Function words a an and as for in of
  the to
• About 400 words in English
• Other frequent words Lotus in a Lotus Support
  

21
Stemming
Feature Space Reduction

• Group morphological variants
• Plural streets street
• Adverbs fully full
• Other inflected word forms goes go
• Grouping process is called conflation
• Current stemming algorithms make mistakes
• Conflating terms manually is difficult
  time-consuming
• Automatic conflation using rules
• Porter Stemmer
• Porter stemming example police policy
  polic

22
Information Gain
Feature Space Reduction

• Measuring information obtained by presence or
  absence of a term in a document
• Feature space reduction by thresholding
• Biased to common term large reduction in size
  of data set cannot be achieved

23
Natural Language Processing
Feature Space Reduction

• Pick out the important words from a document
• For example nouns proper nouns or verbs
• Ignoring all other parts
• Not biased to common terms reduction in bath
  feature space and size of data
• Named entities
• The subset of proper nouns consisting of people
  locations and organization
• Effective in cases of news story classification

24
Experimental Results
Robert Cooley Classification of News Stories
Using Support Vector Machines Proceedings of the
16th International Joint Conference on Artificial
Intelligence Text Mining Workshop 1999

• Data set
• From six news media sources
• Two print sources (New York Times and Associated
  Press Wire)
• Two television sources (ABC World News Tonight
  and CNN Headline News)
• Two radio sources (Public Radio International and
  Voice of America)

25
Experimental Results
Robert Cooley Classification of News Stories
Using Support Vector Machines Proceedings of the
16th International Joint Conference on Artificial
Intelligence Text Mining Workshop 1999

• Results
• NLP significant loss in recall and precision
• SVM kNN (using full text or information gain)
• Binary weighting significant loss in recall

26
kNN
Classification Methods

• Stands for k-nearest neighbor classification
• Algorithms
• Given a test document
• Find k nearest neighbors among training documents
• Calculate and sort score of candidate categories
• Thresholding on these scores
• Decision rule

27
LLSF
Classification Methods

• Stands for Linear Least Squares Fit
• Obtain matrix of word-category regression
  coefficients by LLSF
• FLS arbitrary document vector of weighted
  categories
• By thresholding like kNN assign categories

28
Naïve Bayes
Classification Methods

• Assumption
• Words are drawn randomly from class dependent
  lexicons (with replacement)
• Word independence
• Result

Word independence
Classification rule
29
Estimating the Parameters
Naïve Bayes

• Count frequencies in training data
• Estimating P(Y)
• Fraction of positive / negative examples in
  training data
• Estimating P(WY)
• Smoothing with Laplace estimate

30
Experiment Results
Yiming Yang and Xin Liu A re-examination of text
categorization methods Proceedings of ACM SIGIR
Conference on Research and Development in
Information Retrieval 1999.
31
Text Classification using SVM
T. Joachims A Statistical Learning Model of Text
Classification with Support Vector
Machines Proceedings of the Conference on
Research and Development in Information Retrieval
(SIGIR) ACM 2001.

• A statistical learning model of text
  classification with SVMs

0 if linearly separable
32
Properties 12 Sparse Examples in High Dimension
T. Joachims A Statistical Learning Model of Text
Classification with Support Vector
Machines Proceedings of the Conference on
Research and Development in Information Retrieval
(SIGIR) ACM 2001.

• High dimensional feature vectors (30000
  features)
• Sparse document vectors only a few words of the
  whole language occur in each document
• SVMs use overfitting protection which does not
  depend on the dimension of feature

33
Property 3 Heterogeneous Use of Words
T. Joachims A Statistical Learning Model of Text
Classification with Support Vector
Machines Proceedings of the Conference on
Research and Development in Information Retrieval
(SIGIR) ACM 2001.
No pair of documents shares any words but it
the and of for an a not
that in.
34
Property 4 High Level of Redundancy
T. Joachims A Statistical Learning Model of Text
Classification with Support Vector
Machines Proceedings of the Conference on
Research and Development in Information Retrieval
(SIGIR) ACM 2001.
Few features are irrelevant! Feature space
reduction causes loss of information
35
Property 5 Zipfs Law
T. Joachims A Statistical Learning Model of Text
Classification with Support Vector
Machines Proceedings of the Conference on
Research and Development in Information Retrieval
(SIGIR) ACM 2001.
Most words occur very infrequently!
36
TCat Concepts
T. Joachims A Statistical Learning Model of Text
Classification with Support Vector
Machines Proceedings of the Conference on
Research and Development in Information Retrieval
(SIGIR) ACM 2001.
Modeling real text-classification tasks Used for
previous proof
TCat(2020100 high freq.
412001420055600. medium freq.
91300019300010104000 low
freq. )
37
TCat Concepts
T. Joachims A Statistical Learning Model of Text
Classification with Support Vector
Machines Proceedings of the Conference on
Research and Development in Information Retrieval
(SIGIR) ACM 2001.

• Margin of Tcat-Concepts
• By Zipfs law we can bound R2
• Intuitively many words with low frequency
• relatively short document vectors

with
Linearly separable
38
TCat Concepts
T. Joachims A Statistical Learning Model of Text
Classification with Support Vector
Machines Proceedings of the Conference on
Research and Development in Information Retrieval
(SIGIR) ACM 2001.

• Bound on Expected Error of SVM

39
Text Classification using TSVM
T. Joachims Transductive Inference for Text
Classification using Support Vector
Machines Proceedings of the International
Conference on Machine Learning (ICML) 1999.

• How would you classify the test set
• Training set D1 D6
• Test set D2 D3 D4 D5

40
Why Does Adding Test Examples Reduce Error
T. Joachims Transductive Inference for Text
Classification using Support Vector
Machines Proceedings of the International
Conference on Machine Learning (ICML) 1999.
41
Experiment Results
T. Joachims Transductive Inference for Text
Classification using Support Vector
Machines Proceedings of the International
Conference on Machine Learning (ICML) 1999.

• Data set
• Reuter-21578 dataset-ModApte
• Training 9603 test 3299
• WebKB collection of WWW pages
• Only the class course faculty project
  student are used
• Stemming and stopword removal are not used
• Ohsumed corpus compiled by William Hersh
• Training 10000 test 10000

42
Experiment Results
T. Joachims Transductive Inference for Text
Classification using Support Vector
Machines Proceedings of the International
Conference on Machine Learning (ICML) 1999.

• Results

P/R-breakeven point for Reuters categories
43
Experiment Results
T. Joachims Transductive Inference for Text
Classification using Support Vector
Machines Proceedings of the International
Conference on Machine Learning (ICML) 1999.

• Results

Average P/R-breakeven point on WebKB
Average P/R-breakeven point on Ohsumed
44
Hierarchical Text Classification

• Real world classification complex hierarchical
  structure
• Due to difficulties of training for many classes
  or features

Class 1-1
Class 1
Class 1-2
documents

Class 2
Class 1-3
Class 2-1
Class 3


Level 1
Level 2
45
Hierarchical Text Classification

• More accurate specialized classifiers

computer not discriminating
Hardware
documents
Computers
Software
Chat
Sports
Soccer
Football
computer discriminating
46
Experiment Setting
S. Dumais and H. Chen Hierarchical
classification of Web content. Proceedings of
SIGIR00 August 2000 pp. 256-263.

• Data set LookSmarts web directory
• Using short summary from search engine
• 370597 unique pages
• 17173 categories
• 7-level hierarchy
• Focus on 13 top-level and 150 second-level
  categories

47
Experiment Setting
S. Dumais and H. Chen Hierarchical
classification of Web content. Proceedings of
SIGIR00 August 2000 pp. 256-263.

• Using SVM
• Posterior probabilities by regularized maximum
  likelihood fitting
• Combining probabilities from the first and second
  level
• Boolean scoring function P(L1) P(L2) or
• Multiplicative scoring function P(L1) P(L2)

48
Experiment Results
S. Dumais and H. Chen Hierarchical
classification of Web content. Proceedings of
SIGIR00 August 2000 pp. 256-263.

• Non-hierarchical (baseline) F1 0.476
• Hierarchical
• Top-level
• Training set F1 0.649
• Test set F1 0.572
• Second-level
• Multiplicative F1 0.495
• Boolean F1 0.497
• Assuming top-level classification is correct
• F1 0.711

49
Summary

• Feature space reduction
• Performance of SVM and TSVM is better than others
• TSVM has merits in text classification
• Hierarchical classification is helpful
• Other issues
• Sampling strategies
• Other kinds of feature selection

50
Reference

• T. Joachims Text Categorization with Support
  Vector Machines Learning with Many Relevant
  Features. Proceedings of the European Conference
  on Machine Learning (ECML) Springer 1998.
• T. Joachims Transductive Inference for Text
  Classification using Support Vector Machines.
  Proceedings of the International Conference on
  Machine Learning (ICML) 1999.
• T. Joachims A Statistical Learning Model of Text
  Classification with Support Vector Machines.
  Proceedings of the Conference on Research and
  Development in Information Retrieval (SIGIR)
  ACM 2001.
• Robert Cooley Classification of News Stories
  Using Support Vector Machines (1999). Proceedings
  of the Sixteenth International Joint Conference
  on Artificial Intelligence Text Mining Workshop
  August 1999.
• Yiming Yang and Xin Liu A re-examination of text
  categorization methods. Proceedings of ACM SIGIR
  Conference on Research and Development in
  Information Retrieval (SIGIR) 1999.
• S. Dumais and H. Chen Hierarchical
  classification of Web content. Proceedings of
  SIGIR00 August 2000 pp. 256-263.