Ch4-Pattern_Classification

Ch4-Pattern_Classification - Speech Recognition Pattern...

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Unformatted text preview: Speech Recognition Pattern Classification 2/13/12 Veton Këpuska 2 Pattern Classification u Introduction u Parametric classifiers u Semi-parametric classifiers u Dimensionality reduction u Significance testing 2/13/12 Veton Këpuska 3 Pattern Classification u Goal: To classify objects (or patterns) into categories (or classes) u Types of Problems: 1. Supervised : Classes are known beforehand, and data samples of each class are available 2. Unsupervised : Classes (and/or number of classes) are not known beforehand, and must be inferred from data Feature Extraction Classifier Class i Feature Vectors x Observation s 2/13/12 Veton Këpuska 4 Probability Basics u Discrete probability mass function (PMF): P ( i ω ) u Continuous probability density function (PDF): p(x) u Expected value: E(x) ∑ = i i P 1 ) ( ϖ ∫ ∞ ∞- = 1 ) ( dx x p ∫ ∞ ∞- = dx x xp x E ) ( ) ( 2/13/12 Veton Këpuska 5 Kullback-Liebler Distance u Can be used to compute a distance between two probability mass distributions, P ( zi ), and Q ( zi) u Makes use of inequality log x ≤ x - 1 u Known as relative entropy in information theory u The divergence of P ( zi ) and Q ( zi) is the symmetric sum ( 29 ( 29 ( 29 ( 29 log || ≥ = ∑ i i i i z Q z P z Q Q P D ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ∑ ∑ ∑- = - ≤ i i i i i i i i i i i z Q z P z Q z P z Q z Q z P z Q 1 log ( 29 ( 29 P Q D Q P D || || + 2/13/12 Veton Këpuska 6 Bayes Theorem u Define: {w i} a set of M mutually exclusive classes P(w i) a priori probability for class w i p( x |w i) PDF for feature vector x in class w i P(w i| x ) A posteriori probability of w i given x 2/13/12 Veton Këpuska 7 Bayes Theorem Bayes Rule: From Bayes Rule: Where: ) ( ) ( ) | ( ) | ( x p P x p x P i i i ϖ ϖ ϖ = ∑ = = M i i i P x p x p 1 ) ( ) | ( ) ( ϖ ϖ ) ( ) | ( ) ( ) | ( i i i P x p x p x P ϖ ϖ ϖ = Bayesian Decision Theory Reference: Pattern Classification – R. Duda, P. Hard & D. Stork, Wiley & Sons, 2001 2/13/12 Veton Këpuska 9 Bayes Decision Theory u The probability of making an error given x is: P(error|x)=1-P( w i|x) if decide class w i u To minimize P ( error | x ) (and P ( error )): Choose w i if P(w i|x)>P(w j|x) j ≠i ∀ 2/13/12 Veton Këpuska 10 Bayes Decision Theory u For a two class problem this decision rule means: Choose w 1 if else w 2 u This rule can be expressed as a likelihood ratio: ) ( ) ( ) | ( ) ( ) ( ) | ( 2 2 1 1 x p P x p x p P x p ϖ ϖ ϖ ϖ ≥ ) ( ) ( ) | ( ) | ( 1 2 2 1 ϖ ϖ ϖ ϖ P P x p x p ≥ 2/13/12 Veton Këpuska 11 Bayes Risk u Define cost function ij λ and conditional risk R ( i ω | x ): n ij λ is cost of classifying x as i ω when it is really...
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Ch4-Pattern_Classification - Speech Recognition Pattern...

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