IFT6255:%20Information%20Retrieval%20%20Text%20classification

1
IFT6255 Information RetrievalText
classification
2
Overview

• Definition of text classification
• Important processes in classification
• Classification algorithms
• Advantages and disadvantages of algorithms
• Performance comparison of algorithms
• Conclusion

3
Text Classification

• Text classification (text categorization)
• assign documents to one or more predefined
  categories
• 
  classes
• Documents ?
  class1
• class2
• .
• .
• .
• classn

4
Illustration of Text Classification
Science
Sport
Art
5
Applications of Text Classification

• Organize web pages into hierarchies
• Domain-specific information extraction
• Sort email into different folders
• Find interests of users
• Etc.

6
Text Classification Framework
Documents
Preprocessing
Indexing
Feature selection
Applying classification algorithms
Performance measure
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Preprocessing

• Preprocessing
• transform documents into a suitable
  representation for classification task
• Remove HTML or other tags
• Remove stopwords
• Perform word stemming (Remove suffix)

8
Indexing

• Indexing by different weighing schemes
• Boolean weighing
• Word frequency weighing
• tfidf weighing
• ltc weighing
• Entropy weighing

9
Feature Selection

• Feature selection
• remove non-informative terms from documents
• gtimprove classification effectiveness
• gtreduce computational complexity

10
Different Feature Selection Methods

• Document Frequency Thresholding (DF)
• tf gt threshold
• tfidf
• Information Gain (IG)

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Different Feature Selection Methods

• ?2statistic (CHI)
• or
• A w and Cj B w and not Cj
• C not w and Cj D not w and not Cj
• Mutual Information (MI)

12
Classification Algorithms

• Rocchios algorithm
• K-Nearest-Neighbor algorithm (KNN)
• Decision Tree algorithm (DT)
• Naive Bayes algorithm (NB)
• Artificial Neural Network (ANN)
• Support Vector Machine (SVM)
• Voting algorithms

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Rocchios Algorithm

• Build prototype vector for each class
• prototype vector average vector over all
  training document vectors that belong to class ci
  
• Calculate similarity between test document and
  each of prototype vectors
• Assign test document to the class with maximum
  similarity

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Analysis of Rocchios Algorithm

• Advantages
• Easy to implement
• Very fast learner
• Relevance feedback mechanism
• Disadvantages
• Low classification accuracy
• Linear combination too simple for classification
• Constant ? and ? are empirical

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K-Nearest-Neighbor Algorithm

• Principle points (documents) that are close in
  the space belong to the same class

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K-Nearest-Neighbor Algorithm

• Calculate similarity between test document and
  each neighbor
• Select k nearest neighbors of a test document
  among training examples
• Assign test document to the class which contains
  most of the neighbors

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Analysis of KNN Algorithm

• Advantages
• Effective
• Non-parametric
• More local characteristics of document are
  considered comparing with Rocchio
• Disadvantages
• Classification time is long
• Difficult to find optimal value of k

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Decision Tree Algorithm

• Decision tree associated with document
• Root node contains all documents
• Each internal node is subset of documents
  separated according to one attribute
• Each arc is labeled with predicate which can be
  applied to attribute at parent
• Each leaf node is labeled with a class

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Decision Tree Algorithm

• Recursive partition procedure from root node
• Set of documents separated into subsets according
  to an attribute
• Use the most discriminative attribute first
  (highest IG)
• Pruning to deal with overfitting

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Analysis of Decision Tree Algorithm

• Advantages
• Easy to understand
• Easy to generate rules
• Reduce problem complexity
• Disadvantages
• Training time is relatively expensive
• A document is only connected with one branch
• Once a mistake is made at a higher level, any
  subtree is wrong
• Does not handle continuous variable well
• May suffer from overfitting

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Naïve Bayes Algorithm

• Estimate the probability of each class for a
  document
• Compute the posterior probability (Bayes rule)
• Assumption of word independency

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Naïve Bayes Algorithm

• P(Ci)
• P(djci)

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Analysis of Naïve Bayes Algorithm

• Advantages
• Work well on numeric and textual data
• Easy to implement and computation comparing with
  other algorithms
• Disadvantages
• Conditional independence assumption is violated
  by real-world data, perform very poorly when
  features are highly correlated

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Basic Neuron Model In A Feedforward Network

• Inputs xi arrive through pre-synaptic connections
• Synaptic efficacy is modeled using real weights
  wi
• The response of the neuron is a nonlinear
  function f of its weighted inputs

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Inputs To Neurons

• Arise from other neurons or from outside the
  network
• Nodes whose inputs arise outside the network are
  called input nodes and simply copy values
• An input may excite or inhibit the response of
  the neuron to which it is applied, depending upon
  the weight of the connection

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Weights

• Represent synaptic efficacy and may be excitatory
  or inhibitory
• Normally, positive weights are considered as
  excitatory while negative weights are thought of
  as inhibitory
• Learning is the process of modifying the weights
  in order to produce a network that performs some
  function

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Output

• The response function is normally nonlinear
• Samples include
• Sigmoid
• Piecewise linear

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Backpropagation Preparation

• Training SetA collection of input-output
  patterns that are used to train the network
• Testing SetA collection of input-output patterns
  that are used to assess network performance
• Learning Rate-?A scalar parameter, analogous to
  step size in numerical integration, used to set
  the rate of adjustments

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Network Error

• Total-Sum-Squared-Error (TSSE)
• Root-Mean-Squared-Error (RMSE)

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A Pseudo-Code Algorithm

• Randomly choose the initial weights
• While error is too large
• For each training pattern
• Apply the inputs to the network
• Calculate the output for every neuron from the
  input layer, through the hidden layer(s), to the
  output layer
• Calculate the error at the outputs
• Use the output error to compute error signals for
  pre-output layers
• Use the error signals to compute weight
  adjustments
• Apply the weight adjustments
• Periodically evaluate the network performance

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Apply Inputs From A Pattern

• Apply the value of each input parameter to each
  input node
• Input nodes computer only the identity function

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Calculate Outputs For Each Neuron Based On The
Pattern

• The output from neuron j for pattern p is Opj
  where
• and
• k ranges over the input indices and Wjk is the
  weight on the connection from input k to neuron j

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Calculate The Error Signal For Each Output Neuron

• The output neuron error signal dpj is given by
  dpj(Tpj-Opj) Opj (1-Opj)
• Tpj is the target value of output neuron j for
  pattern p
• Opj is the actual output value of output neuron j
  for pattern p

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Calculate The Error Signal For Each Hidden Neuron

• The hidden neuron error signal dpj is given by
• where dpk is the error signal of a post-synaptic
  neuron k and Wkj is the weight of the connection
  from hidden neuron j to the post-synaptic neuron
  k

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Calculate And Apply Weight Adjustments

• Compute weight adjustments DWji byDWji ? dpj
  Opi
• Apply weight adjustments according to Wji Wji
  DWji

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Analysis of ANN Algorithm

• Advantages
• Produce good results in complex domains
• Suitable for both discrete and continuous data
  (especially better for the continuous domain)
• Testing is very fast
• Disadvantages
• Training is relatively slow
• Learned results are difficult for users to
  interpret than learned rules (comparing with DT)
• Empirical Risk Minimization (ERM) makes ANN try
  to minimize training error, may lead to
  overfitting

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Support Vector Machines

• Main idea of SVMs
• Find out the linear separating hyperplane which
  maximize the margin, i.e., the optimal separating
  hyperplane (OSH)
• Nonlinear separable case
• Kernel function and Hilbert space

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SVM classification
Maximizing the margin is equivalent to
Introducing Lagrange multipliers , the
Lagrangian is
Dual problem
subject to
The solution is given by
The problem of classifying a new data point x is
now simply solved by looking at the sigh of
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Analysis of SVM Algorithm

• Advantages
• Comparing with ANN, SVM capture the inherent
  characteristics of the data better
• Embedding the Structural Risk Minimization (SRM)
  principle which minimizes the upper bound on the
  generalization error (better than the Empirical
  Risk Minimization principle)
• Ability to learn can be independent of the
  dimensionality of the feature space
• Global minima vs. local minima
• Disadvantage
• Parameter tuning
• kernel selection

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Voting Algorithm

• Principle using multiple evidence (multiple poor
  classifiersgt single good classifier)
• Generate some base classifiers
• Combine them to make the final decision

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Bagging Algorithm

• Use multiple versions of a training set D of size
  N, each created by resampling N examples from D
  with bootstrap
• Each of data sets is used to train a base
  classifier, the final classification decision is
  made by the majority voting of these classifiers

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Adaboost

• Main idea
• The main idea of this algorithm is to maintain a
  distribution or set of weights over the training
  set. Initially, all weights are set equally, but
  in each iteration the weights of incorrectly
  classified examples are increased so that the
  base classifier is forced to focus on the hard
  examples in the training set. For those correctly
  classified examples, their weights are decreased
  so that they are less important in next
  iteration.

• Why ensembles can improve performance
• Uncorrelated errors made by the individual
  classifiers can be removed by voting.
• Our hypothesis space H may not contain the true
  function f. Instead, H may include several
  equally good approximations to f. By taking
  weighted combinations of these approximations, we
  may be able to represent classifiers that lie
  outside of H.

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Adaboost algorithm
Given m examples
where
Initialize
for all i 1m

• For t 1,,T
• Train base classifier using distribution

• Get a hypothesis

with error

• Choose

.

• Update

where
is a normalization factor (chosen so that
will be a distribution).
Output the final hypothesis
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Analysis of Voting Algorithms

• Advantage
• Surprisingly effective
• Robust to noise
• Decrease the overfitting effect
• Disadvantage
• Require more calculation and memory

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Performance Measure

• Performance of algorithm
• Training time
• Testing time
• Classification accuracy
• Precision, Recall
• Micro-average / Macro-average
• Breakeven precision recall
• Goal high classification quality and
  computation efficiency

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Comparison Based on Six Classifiers

• Classification accuracy six classifiers
  (Reuters-21578 collection)

    1 2 3 4
  Author Dumais Joachims Weiss Yang
1 Training 9603 9603 9603 7789
2 Test 3299 3299 3299 3309
3 Topics 118 90 95 93
4 Indexing Boolean tfc Frequency ltc
5 Selection MI IG - ?2
7 Measure Breakeven Microavg. Breakeven Breakeven
8 Rocchio 61.7 79.9 78.7 75
9 NB 75.2 72 73.4 71
10 KNN N/A 82.3 86.3 85
11 DT N/A 79.4 78.9 79
12 SVM 87 86 86.3 N/A
13 Voting N/A N/A 87.8 N/A
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Analysis of Results

• SVM, Voting and KNN are showed good performance
• DT, NB and Rocchio showed relatively poor
  performance

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Comparison Based on Feature Selection

• Classification accuracy NB vs. KNN vs. SVM
  (Reuter collection)

of features NB KNN SVM
10 48.66 0.10 57.31 0.2 60.78 0.17
20 52.28 0.15 62.57 0.16 73.67 0.11
40 59.19 0.15 68.39 0.13 77.07 0.14
50 60.32 0.14 74.22 0.11 79.02 0.13
75 66.18 0.19 76.41 0.11 83.0 0.10
100 77.9 0.19 80.2 0.09 84.3 0.12
200 78.26 0.15 82.5 0.09 86.94 0.11
500 80.80 0.12 82.19 0.08 86.59 0.10
1000 80.88 0.11 82.91 0.07 86.31 0.08
5000 79.26 0.07 82.97 0.06 86.57 0.04
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Analysis of Results

• Accuracy is improved with an increase in the
  number of features until some level
• Top level approximately 500-1000 features
  accuracy reaches its peak and begins to decline
• SVM obtains the best performance

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Conclusion

• Different algorithms perform differently
  depending on data collections
• Some algorithms (e.g. Rocchio) do not perform
  well
• None of them appears to be globally superior over
  the others however, SVM and Voting are good
  choices by considering all the factors