This presentation is about Multiple Classifier System (Ensemble of Classifiers). At first tell about the general idea of decision making, then address reasons and rationales of using Multiple Classifier System, after that concentrate on designing Multiple Classifier System: 1.Create an Ensemble 2.Combining Classifiers.
2. Outline
Introduction
Decision Making
General Idea
Brief History
Reasons & Rationale
Statistical
Large volumes of data
Too little data
Divide and Conquer
Data Fusion
Multiple Classifier system
Designing
Diversity
Create an Ensemble
Combining Classifiers
Example
Conclusions
References
3. Ensemble-based Systems in Decision
Making
For many tasks, we often seek second opinion before making a decision,
sometimes many more
Consulting different doctors before a major surgery
Reading reviews before buying a product
Requesting references before hiring someone
We consider decisions of multiple experts in our daily lives
Why not follow the same strategy in automated decision making?
Multiple classifier systems, committee of classifiers, mixture of experts,
ensemble based systems
4. Ensemble-based Classifiers
How to (i) generate individual components of the ensemble systems
(base classifiers), and (ii) how to combine the outputs of individual
classifiers?
5. Brief History of Ensemble Systems
Dasarathy and Sheela (1979) partitioned the feature space using two
or more classifiers
Schapire (1990) proved that a strong classifier can be generated by
combining weak classifiers through boosting; predecessor of AdaBoost
algorithm
Two types of combination:
classifier selection
classifier fusion
7. Why Ensemble Based Systems?
1. Statistical reasons
A set of classifiers with similar training performances may have
different generalization performances
Combining outputs of several classifiers reduces the risk of
selecting a poorly performing classifier
Example:
Suppose there are 25 base classifiers
Each classifier has error rate, = 0.35
Probability that the ensemble classifier makes a wrong prediction:
25
25
1
25
(1 ) 0.06 i i
i i
8. Why Ensemble Based Systems?
2. Large volumes of data
If the amount of data to be analyzed is too large, a single classifier
may not be able to handle it; train different classifiers on
different partitions of data
9. Why Ensemble Based Systems?
3. Too little data
Ensemble systems can also be used when there is too little data;
resampling techniques
10. Why Ensemble Based Systems?
4. Divide and Conquer
Divide data space into smaller & easier-to-learn partitions; each
classifier learns only one of the simpler partitions
11. Why Ensemble Based Systems?
5. Data Fusion
Given several sets of data from various sources, where the nature
of features is different (heterogeneous features), training a single
classifier may not be appropriate (e.g., MRI data, EEG recording,
blood test,..)
13. Major Steps
All ensemble systems must have two key components:
Generate component classifiers of the ensemble
Method for combining the classifier outputs
14. “Diversity” of Ensemble
Objective: create many classifiers, and combine their outputs
to improve the performance of a single classifier
Intuition: if each classifier makes different errors, then their
strategic combination can reduce the total error!
Need base classifiers whose decision boundaries are adequately
different from those of others
Such a set of classifiers is said to be “diverse”
15. How to achieve classifier diversity?
A. Use different training sets to train individual classifiers
B. Use different training parameters for a classifier
C. Different types of classifiers (MLPs, decision trees, NN
classifiers, SVM) can be combined for added diversity
D. Using random feature subsets, called random subspace
method
22. Two Important Concept (i)
(i) trainable vs. non-trainable
Trainable rules: parameters of the combiner, called
“weights” determined through a separate training algorithm
Non-trainable rules: combination parameters are
available as classifiers are generated; Weighted majority
voting is an example
23. Two Important Concept (ii)
(ii) Type of the output of classifiers
Combine
Classifier
Absolute
output
Majority Voting
Naïve Bayes
Behavior
Knowledge Space
Ranked
output
Borda Counting
Maximum
Ranking
Continuous
output
Algebraic
Metohd
Fuzzy Integral
Decesion
Template
25. Conclusions
Ensemble systems are useful in practice
Diversity of the base classifiers is important
Ensemble generation techniques: bagging, AdaBoost, mixture of
experts
Classifier combination strategies: algebraic combiners, voting
methods, and decision templates.
No single ensemble generation algorithm or combination rule is
universally better than others
Effectiveness on real world data depends on the classifier diversity
and characteristics of the data
26. References
[1] Polikar R., “Ensemble Based Systems in Decision Making,” IEEE
Circuits and Systems Magazine, vol.6, no. 3, pp. 21-45, 2006
[2] Polikar R., “Bootstrap Inspired Techniques in Computational
Intelligence,” IEEE Signal Processing Magazine, vol.24, no. 4, pp. 56-
72, 2007
[3] Polikar R., “Ensemble Learning,” Scholarpedia, 2008.
[4] Kuncheva, L. I. , Combining Pattern Classifiers: Methods and
Algorithms. New York, NY: Wiley, 2004.