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March 2006Alon Slapak 1 of 1 Bayes Classification A practical approach Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 2 of 2 Discriminant function Definition: a discriminant function is an n-dimensional hypersurface which divides the n-dimensional feature space into two separate areas contain separate classes. A 2-dimwnsinal discriminant function A 1-dimwnsinal discriminant function 2-dimensional feature space 1-dimensional feature space Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 3 of 3 Discriminant function Let h(x) be a discriminant function. A two-category classifier uses the following rule: Decide 1 if h(x) > 0 and 2 if h(x) < 0 If h(x) = 0 x is assigned to either class. h(x)=x h(x) < 0 Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 4 of 4 Thomas Bayes At the time of his death, Rev. Thomas Bayes (1702 –1761) left behind two unpublished essays attempting to determine the probabilities of causes from observed effects. Forwarded to the British Royal Society, the essays had little impact and were soon forgotten. When several years later, the French mathematician Laplace independently rediscovered a very similar concept, the English scientists quickly reclaimed the ownership of what is now known as the “Bayes Theorem”. Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 5 of 5 Conditional Probability Definition: Let A and B be events with P(B) > 0. The conditional probability of A given B, denoted by P(A|B), is defined as: P(A|B) = P(A B)/P(B) A B Venn Diagram Given: N(A) = 30 N(A B) = 10 P(B | A) = N(A B)/N(A) = 10/30 = 1/3 Example: Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 6 of 6 Bayes’ theorem Since P(A | B) = P(A B)/P(B), we have:P(A | B)P(B) = P(A B) Symmetrically we have:P(B | A)P(A) = P(B A) = P(A B) Therefore: P(A | B)P(B) = P(B | A)P(A) And: where P(A | B) is the conditional probability, P(A), P(B) are the prior probabilities, P(B | A) is the posterior probability Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 7 of 7 Bayes’ theorem in a pattern recognition notation Given classes i and a pattern x, prior probability The prior probability reflects knowledge of the relative frequency of instances of a class likelihood The likelihood is a measure of the probability that a measurement value occurs in a class. evidence The evidence is a scaling term Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 8 of 8 Bayes classifier The following phrase classify each pattern x to one of two classes: or (since P(x) is common to both sides): Means, decide 1 if P( 1 |x) > P( 2 |x) Likelihood ratio Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 9 of 9 Bayes Discriminant function Since a ratio of probabilities may yield very small values, it is common to use the log of the likelihood ratio: and the derived Bayes’ discriminant function is: Remember: Decide 1 if h(x) > 0 and 2 if h(x) < 0 If h(x) = 0 x is assigned to either class. Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 10 of 10 Example - Gaussian Distributions A multi dimensional Gaussian distribution is: Females Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 11 of 11 Example - Gaussian Distributions A multi dimensional Gausian distribution is: Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 12 of 12 Example - Gaussian Distributions Assume two Gaussian distributed classes with And clear all N1 = 150; N2 = 150; E1 = [50 40; 40 50]; E2 = [50 40; 40 50]; M1 = [30,55]'; M2 = [60,40]'; % % Classes drawing % [P1,A1] = eig(E1); [P2,A2] = eig(E2); y1=randn(2,N1); y2=randn(2,N2); for i=1:N1, x1(:,i) =P1*sqrt(A1)* y1(:,i)+M1; end; for i=1:N2, x2(:,i) =P2*sqrt(A2)* y2(:,i)+M2; end; figure; plot(x1(1,:),x1(2,:),'^',x2(1,:),x2(2,:),'or'); axis([ ]); xlabel('x1') ylabel('x2') Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 13 of 13 Example - Gaussian Distributions % % Classifier drawing % ep=1.2; k=1; for i=0:k:100, for j=0:k:100, x=([i;j]); h=0.5*(x-M1)'*inv(E1)*(x-M1)-0.5*(x-M2)'*inv(E2)*(x-M2)+0.5*log(det(E1)/det(E2)); if (abs(h)

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March 2006Alon Slapak 14 of 14 Example - Gaussian Distributions Assume two Gaussian distributed classes with And clear all N1 = 150; N2 = 150; E1 = [50 40; 40 50]; E2 = [50 -40; ]; M1 = [30,55]'; M2 = [60,40]'; % % Classes drawing % [P1,A1] = eig(E1); [P2,A2] = eig(E2); y1=randn(2,N1); y2=randn(2,N2); for i=1:N1, x1(:,i) =P1*sqrt(A1)* y1(:,i)+M1; end; for i=1:N2, x2(:,i) =P2*sqrt(A2)* y2(:,i)+M2; end; figure; plot(x1(1,:),x1(2,:),'^',x2(1,:),x2(2,:),'or'); axis([ ]); xlabel('x1') ylabel('x2') Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 15 of 15 Example - Gaussian Distributions % % Classifier drawing % ep=1; k=1; for i=0:k:100, for j=0:k:100, x=([i;j]); h=0.5*(x-M1)'*inv(E1)*(x-M1)-0.5*(x-M2)'*inv(E2)*(x-M2)+0.5*log(det(E1)/det(E2)); if (abs(h)

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March 2006Alon Slapak 16 of 16 Exercise Synthesize two classes with different a- priory probabilities. Show how the probabilities influence the discriminant function. Synthesize three classes and plot the discriminant functions. Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 17 of 17 Summary Steps for Building a Bayesian Classifier Collect class exemplars Estimate class a priori probabilities Estimate class means Form covariance matrices, find the inverse and determinant for each Form the discriminant function for each class Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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March 2006Alon Slapak 18 of 18 Bibliography 1.K. Fukunaga, Introduction to Statistical Pattern Recognition, 2 nd ed., Academic Press, San Diego, L. I. Kuncheva, J. C. Bezdek amd R. P.W. Duin, “Decision Templates for Multiple Classier Fusion: An Experimental Comparison”, Pattern Recognition, 34, (2), pp , R. O. Duda, P. E. Hart and D. G. Stork, Pattern Classification (2nd ed), John Wiley & Sons, Example Discriminant function Bayes theorem Bayes discriminant function Bibliography

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