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# class20 - 9.520 Class 20 Bayesian Interpretations Tomaso...

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Unformatted text preview: 9.520: Class 20 Bayesian Interpretations Tomaso Poggio and Sayan Mukherjee Plan • Bayesian interpretation of Regularization • Bayesian interpretation of the regularizer • Bayesian interpretation of quadratic loss • Bayesian interpretation of SVM loss • Consistency check of MAP and mean solutions for quadratic loss • Synthesizing kernels from data: bayesian foundations • Selection (called “alignment”) as a special case of kernel synthesis Bayesian Interpretation of RN, SVM, and BPD in Regression Consider min f ∈H 1 ‘ ‘ X i =1 ( y i- f ( x i )) 2 + λ k f k 2 K We will show that there is a Bayesian interpretation of RN in which the data term – that is the term with the loss function – is a model of the noise and the stabilizer is a prior on the hypothesis space of functions f . Definitions 1. D ‘ = { ( x i , y i ) } for i = 1 , ··· , ‘ is the set of training examples 2. P [ f | D ‘ ] is the conditional probability of the function f given the examples g . 3. P [ D ‘ | f ] is the conditional probability of g given f , i.e. a model of the noise. 4. P [ f ] is the a priori probability of the random field f . Posterior Probability The posterior distribution P [ f | g ] can be computed by ap- plying Bayes rule: P [ f | D ‘ ] = P [ D ‘ | f ] P [ f ] P ( D ‘ ) . If the noise is normally distributed with variance σ , then the probability P [ D ‘ | f ] is P [ D ‘ | f ] = 1 Z L e- 1 2 σ 2 ∑ ‘ i =1 ( y i- f ( x i )) 2 where Z L is a normalization constant. Posterior Probability Informally (we will make it precise later), if P [ f ] = 1 Z r e-k f k 2 K where Z r is another normalization constant, then P [ f | D ‘ ] = 1 Z D Z L Z r e- 1 2 σ 2 ∑ ‘ i =1 ( y i- f ( x i )) 2 + k f k 2 K MAP Estimate One of the several possible estimates of f from P [ f | D ‘ ] is the so called MAP estimate, that is max P [ f | D ‘ ] = min ‘ X i =1 ( y i- f ( x i )) 2 + 2 σ 2 k f k 2 K . which is the same as the regularization functional if λ = 2 σ 2 /‘. Bayesian Interpretation of the Data Term (quadratic loss) As we just showed, the quadratic loss (the standard RN case) corresponds in the Bayesian interpretation to as- suming that the data y i are affected by additive indepen- dent Gaussian noise processes, i.e. y i = f ( x i ) + i with E [ j j ] = 2 δ i,j P ( y | f ) ∝ exp(- X ( y i- f ( x i )) 2 ) Bayesian Interpretation of the Data Term (nonquadratic loss) To find the Bayesian interpretation of the SVM loss, we now assume a more general form of noise. We assume that the data are affected by additive independent noise sam- pled form a continuous mixture of Gaussian distributions with variance β and mean μ according to P ( y | f ) ∝ exp- Z ∞ dβ Z ∞-∞ dμ q βe- β ( y- f ( x )- μ ) 2 P ( β, μ ) , The previous case of quadratic loss corresponds to P ( β, μ ) = δ β- 1 2 σ 2 δ ( μ ) . Bayesian Interpretation of the Data Term (absolute loss) To find P ( β, μ ) that yields a given loss function...
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class20 - 9.520 Class 20 Bayesian Interpretations Tomaso...

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