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Unformatted text preview: CSE 6740 Lecture 4 How Do I Learn Any Density? (Nonparametric Estimation) Alexander Gray agray@cc.gatech.edu Georgia Institute of Technology CSE 6740 Lecture 4 p. 1/3 6 Today 1. Nonparametric estimation (What if I dont want to specify a simple parametric form?) 2. Kernel density estimation (How can I estimate a density nonparametrically?) CSE 6740 Lecture 4 p. 2/3 6 Nonparametric Estimation What if I dont want to specify a simple parametric form? CSE 6740 Lecture 4 p. 3/3 6 Nonparametric Estimation What exactly do we mean by nonparametric? Example of a nonparametric model class, called a Sobolev space : F = braceleftbigg f : integraldisplay ( f ( x )) 2 dx < bracerightbigg (1) Nonparametric doesnt mean there are no parameters. There is typically a local model. It refers to model classes, like the one above, which arent parametric (having finite number of parameters). We sometimes say such a class is distributionfree . CSE 6740 Lecture 4 p. 4/3 6 Nonparametric Estimation A nonparametric method is one for which we can pretend the model class is actually such a class, as far as its asymptotic properties. In other words, it is a method for which one can show something like consistency with respect to a very general class of distributions (we want to say any distribution but this is of course never quite true). CSE 6740 Lecture 4 p. 5/3 6 Examples of Nonparametric Methods Some examples of popular nonparametric methods: Histogram, kernel density estimation (density estimation) Splines, wavelet regression (regression) Kernel discriminant analysis, nearest neighbor, support vector machines (classification) CSE 6740 Lecture 4 p. 6/3 6 Histogram Perhaps the simplest nonparametric density estimator is the histogram : hatwide f N ( x ) = m summationdisplay j =1 hatwide p j h I ( x B j ) (2) where h = 1 /m is the binwidth , Y j is the number of observations in bins B 1 = bracketleftbig , 1 m ) ,B 1 = bracketleftbig 1 m , 2 m ) ,... , hatwide p j = Y j /N , and p j = integraltext B j f ( u ) du . CSE 6740 Lecture 4 p. 7/3 6 Histogram CSE 6740 Lecture 4 p. 8/3 6 Histogram CSE 6740 Lecture 4 p. 9/3 6 Histogram Note a few things. First, the placement of the bins ( i.e. shifting a bit to the left or right) can make a significant qualitative difference. Second, the density estimate is not smooth....
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