CSC 411: Lecture 07: Multiclass Classification

Transcription

1 CSC 411: Lecture 07: Multiclass Classification Class based on Raquel Urtasun & Rich Zemel s lectures Sanja Fidler University of Toronto Feb 1, 2016 Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

2 Today Multi-class classification with: Least-squares regression Logistic Regression K-NN Decision trees Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

3 Discriminant Functions for K > 2 classes First idea: Use K 1 classifiers, each solving a two class problem of separating point in a class C from points not in the class. Known as 1 vs all or 1 vs the rest classifier PROBLEM: More than one good answer for green region! Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

4 Discriminant Functions for K > 2 classes Another simple idea: Introduce K(K 1)/2 two-way classifiers, one for each possible pair of classes Each point is classified according to majority vote amongst the disc. func. Known as the 1 vs 1 classifier PROBLEM: Two-way preferences need not be transitive Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

5 K-Class Discriminant We can avoid these problems by considering a single K-class discriminant comprising K functions of the form y (x) = w T x + w,0 and then assigning a point x to class C if j y (x) > y j (x) Note that w T is now a vector, not the -th coordinate The decision boundary between class C j and class C is given by y j (x) = y (x), and thus it s a (D 1) dimensional hyperplane defined as (w w j ) T x + (w 0 w j0 ) = 0 What about the binary case? Is this different? What is the shape of the overall decision boundary? Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

6 K-Class Discriminant The decision regions of such a discriminant are always singly connected and convex In Euclidean space, an object is convex if for every pair of points within the object, every point on the straight line segment that joins the pair of points is also within the object Which object is convex? Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

7 K-Class Discriminant The decision regions of such a discriminant are always singly connected and convex Consider 2 points x A and x B that lie inside decision region R Any convex combination ˆx of those points also will be in R ˆx = λx A + (1 λ)x B Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

8 Proof A convex combination point, i.e., λ [0, 1] ˆx = λx A + (1 λ)x B From the linearity of the classifier y(x) y (ˆx) = λy (x A ) + (1 λ)y (x B ) Since x A and x B are in R, it follows that y (x A ) > y j (x A ), y (x B ) > y j (x B ), j Since λ and 1 λ are positive, then ˆx is inside R Thus R is singly connected and convex Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

9 Example Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

10 Multi-class Classification with Linear Regression From before we have: which can be rewritten as: y (x) = w T x + w,0 y(x) = W T x where the -th column of W is [w,0, w T ]T, and x is [1, x T ] T Training: How can I find the weights W with the standard sum-of-squares regression loss? 1-of-K encoding: For multi-class problems (with K classes), instead of using t = (target has label ) we often use a 1-of-K encoding, i.e., a vector of K target values containing a single 1 for the correct class and zeros elsewhere Example: For a 4-class problem, we would write a target with class label 2 as: t = [0, 1, 0, 0] T Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

11 Multi-class Classification with Linear Regression Sum-of-least-squares loss: l( W) = N W T x (n) t (n) 2 n=1 = X W T 2 F where the n-th row of X is [ x (n) ] T, and n-th row of T is [t (n) ] T Setting derivative wrt W to 0, we get: W = ( X T X) 1 X T T Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

12 Multi-class Logistic Regression Associate a set of weights with each class, then use a normalized exponential output where the activations are given by p(c x) = y (x) = exp(z ) j exp(z j) z = w T x The function exp(z ) j exp(z j ) is called a softmax function Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

13 Multi-class Logistic Regression The lielihood p(t w 1,, w ) = N K n=1 =1 p(c x (n) ) t(n) = N K n=1 =1 with p(c x) = y (x) = exp(z ) j exp(z j) where n-th row of T is 1-of-K encoding of example n and z = w T x + w 0 What assumptions have I used to derive the lielihood? Derive the loss by computing the negative log-lielihood: E(w 1,, w K ) = log p(t w 1,, w K ) = N K n=1 =1 y (n) (x (n) ) t (n) t (n) log[y (n) (x (n) )] This is nown as the cross-entropy error for multiclass classification How do we obtain the weights? Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

14 Training Multi-class Logistic Regression How do we obtain the weights? E(w 1,, w K ) = log p(t w 1,, w K ) = Do gradient descent, where the derivatives are and E w,j = N E z (n) K n=1 j=1 y (n) j z (n) = E K j=1 y (n) j = δ(, j)y (n) j E y (n) j y (n) j z (n) y (n) j z (n) The derivative is the error times the input z(n) = w,j N K n=1 =1 y (n) j y (n) = y (n) N t (n) (y (n) n=1 t (n) log[y (n) (x (n) )] t (n) ) x (n) j Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

15 Softmax for 2 Classes Let s write the probability of one of the classes p(c 1 x) = y 1 (x) = exp(z 1) j exp(z j) = exp(z 1 ) exp(z 1 ) + exp(z 2 ) I can equivalently write this as p(c 1 x) = y 1 (x) = exp(z 1 ) exp(z 1 ) + exp(z 2 ) = exp ( (z 1 z 2 )) So the logistic is just a special case that avoids using redundant parameters Rather than having two separate set of weights for the two classes, combine into one z = z 1 z 2 = w T 1 x w T 2 x = w T x The over-parameterization of the softmax is because the probabilities must add to 1. Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

16 Multi-class K-NN Can directly handle multi class problems Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17

17 Multi-class Decision Trees Can directly handle multi class problems How is this decision tree constructed? Urtasun, Zemel, Fidler (UofT) CSC 411: 07-Multiclass Classification Feb 1, / 17