CS479

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Bayesian Decision Theory. Fundamental statistical approach to problem classification.Quantifies the tradeoffs between various classification decisions using probabilities and the costs associated with such decisions.Each action is associated with a cost or risk.The simplest risk is the classifica

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CS479

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1. CS479/679 Pattern Recognition Spring 2006 – Prof. Bebis Bayesian Decision Theory Chapter 2 (Duda et al.)

2. Bayesian Decision Theory Fundamental statistical approach to problem classification. Quantifies the tradeoffs between various classification decisions using probabilities and the costs associated with such decisions. Each action is associated with a cost or risk. The simplest risk is the classification error. Design classifiers to recommend actions that minimize some total expected risk.

3. Terminology (using sea bass – salmon classification example) State of nature ? (random variable): ?1 for sea bass, ?2 for salmon. Probabilities P(?1) and P(?2 ) (priors) prior knowledge of how likely is to get a sea bass or a salmon Probability density function p(x) (evidence): how frequently we will measure a pattern with feature value x (e.g., x is a lightness measurement) Note: if x and y are different measurements, p(x) and p(y) correspond to different pdfs: pX(x) and pY(y)

4. Terminology (cont’d) (using sea bass – salmon classification example) Conditional probability density p(x/?j) (likelihood): how frequently we will measure a pattern with feature value x given that the pattern belongs to class ?j

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