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Linear Separators

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Bankruptcy example R is the ratio of earnings to expenses L is the number of late payments on credit cards over the past year. We would like here to draw a linear separator, and get so a classifier.

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1-Nearest Neighbor Boundary The decision boundary will be the boundary between cells defined by points of different classes, as illustrated by the bold line shown here.

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Decision Tree Boundary Similarly, a decision tree also defines a decision boundary in the feature space. Although both 1-NN and decision trees agree on all the training points, they disagree on the precise decision boundary and so will classify some query points differently. This is the essential difference between different learning algorithms.

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Linear Boundary Linear separators are characterized by a single linear decision boundary in the space. –The bankruptcy data can be successfully separated in that manner. But, there is no guarantee that a single linear separator will successfully classify any set of training data.

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Linear Hypothesis Class Line equation (assume 2D first): w 2 x 2 +w 1 x 1 +b=0 Fact1: All the points (x 1, x 2 ) lying on the line make the equation true. Fact2: The line separates the plane in two half-planes. Fact3: The points (x 1, x 2 ) in one half-plane give us an inequality with respect to 0, which has the same direction for each of the points in the half-plane. Fact4: The points (x 1, x 2 ) in the other half-plane give us the reverse inequality with respect to 0.

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Fact 3 proof w 2 x 2 +w 1 x 1 +b=0 We can write it as: x1x1 x2x2 0 (p,q) (p,r) (p,r) is on the line so: But q

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Fact 4 proof w 2 x 2 +w 1 x 1 +b=0 We can write it as: (p,r) is on the line so: But s>r, so we get: Since (p,s) was an arbitrary point in the (other) half-plane, we say that the same direction of inequality holds for any other point of that half-plane. x1x1 x2x2 0 (p,r) (p,s) i.e.

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Corollary What’s an easy way to determine the direction of the inequalities for each subplane? –Try it for the point (0,0), and determine the direction for the half- plane where (0,0) belongs. –The points of the other half-plane will have the opposite inequality direction. How much bigger (or smaller) than zero is w 2 p+w 1 q+b is proportional to the distance of the point (p,q) from the line. The same can be said for an n-dimensional space. Simply, we don’t talk about “half-planes” but “half-spaces” (line is now hyperplane creating two half-spaces)

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Linear classifier We can now exploit the sign of this distance to define a linear classifier, one whose decision boundary is a hyperplane. Instead of using 0 and 1 as the class labels (which was an arbitrary choice anyway) we use the sign of the distance, either +1 or -1 as the labels (that is the values of the y i ’s). Which outputs +1 or –1. Trick

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Margin The margin is the product of w.x i for the training point x i and the known sign of the class, y i. margin: i = y i w.x i is proportional to perpendicular distance of point x i to line (hyperplane). i > 0 : point is correctly classified (sign of distance = y i ) i < 0 : point is incorrectly classified (sign of distance y i )

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Perceptron algorithm How to find a linear separator? Perceptron algorithm, was developed by Rosenblatt in the mid 50's. This is a greedy, "mistake driven" algorithm. Algorithm Pick initial weight vector (including b), e.g. [.1, …,.1] Repeat until all points get correctly classified Repeat for each point x i –Calculate margin y i.w.x i (this is number) –If margin > 0, point x i is correctly classified –Else, change weights proportional to y i.x i

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Gradient Ascent/Descent Why pick y i.x i as increment to weights? The margin is a multiple input variable function. –The variables are w 2, w 1, w 0 (or in general w n,…,w 0 ) In order to reach the maximum of this function, it is good to change the variables in the direction of the slope of the function. The slope is represented by the gradient of the function. –The gradient is the vector of first (partial) derivatives of the function with respect to each of the input variables.

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Perceptron algorithm Changes for the different points interfere with each other. –So, it will not be the case that one pass through the points will produce a correct weight vector. –In general, we will have to go around multiple times. However, the algorithm is guaranteed to terminate with the weights for a separating hyperplane as long as the data is linearly separable. –The proof of this fact is beyond our scope. Notice that if the data is not separable, then this algorithm is an infinite loop. –Good idea to keep track of the best separator we've seen so far.

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Perceptron algorithm Bankruptcy data 49 iterations through the bankruptcy data for the algorithm to stop. The separator at the end of the loop is [0.4, 0.94, -2.2] We can pick some small "rate" constant to scale the change to w. This is called eta.

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Dual Form The calculated w will be: where, i is the number of times data instance x i got missclassified. So, for classification we’ll check: where x is the new data instance to e classified.

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Perceptron algorithm = 0 Repeat until all points get correctly classified Repeat for each point x i –Calculate margin –If margin > 0, point x i is correctly classified –Else, increment i. If data is not linearly separable then alphas grow without bound

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Non-linearly separable

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Moving points into a different space Square every x 1 and x 2 value first. –A point that was at (-1,2) would now be at (1,4), –A point that was at (0.5,1) would now be at (0.25,1), and so on. Very easy now to divide X's from O's.

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Main Idea Transform the points (vectors) into another space using some function and then do linear separation in the new space, i.e. considering vectors (x 1 ), (x 2 ),..., (x n ).

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The Kernel Trick While you could write code to transform the data into a new space like this, it isn't usually done in practice because finding a dividing line when working with real datasets can require casting the data into hundreds or thousands of dimensions, and this is quite impractical to implement. However, with any algorithm that uses dot-products—including the linear classifier—you can use a technique called the kernel trick. The kernel trick involves replacing the dot-product function with a new function that returns what the dot-product would have been if the data had first been transformed to a higher dimensional space using some mapping function.

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The Kernel Trick Remember, all we care is computing dot products. See something interesting: Let : R 2 R 3 such that (x) = ([x 1, x 2 ]) = [z 1, z 2, z 3 ] = [x 1 2, 2x 1 x 2, x 2 2 ] Now, let r = [r 1, r 2, r 3 ] and s = [s 1, s 2, s 3 ] be two vectors in R 3 corresponding to vectors a = [a 1, a 2 ] and b = [b 1, b 2 ] in R 2. (a) (b) = r s = r 1 s 1 +r 2 s 2 +r 3 s 3 = (a 1 b 1 ) 2 + 2a 1 a 2 b 1 b 2 + (a 2 b 2 ) 2 = (a 1 b 1 +a 2 b 2 ) 2 = (a b) 2

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The Kernel Trick So instead of mapping the data vectors via and computing the modified inner product (a) (b), we can do it in one operation, leaving the mapping completely implicit. Because “modified inner product” is a long name, we call it a kernel, K(a, b) = (a) (b). Useful Kernels Polynomial Kernel: K(a, b) = (a b) 2 –Visualization: Gaussian Kernel: K(a, b) = e (1/2)||x−y||^2

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Line Separators It's difficult to characterize the separator that the Perceptron algorithm will come up with. Different runs can come up with different separators. Can we do better?

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Which one to pick? Natural choice: Pick the separator that has the maximal margin to its closest points on either side. –Most conservative. –Any other separator will be "closer" to one class than to the other. Those closest points are called "support vectors".

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