Machine Learning Basics Lecture 4: SVM I. Princeton University COS 495 Instructor: Yingyu Liang

Transcription

1 Machine Learning Basics Lecture 4: SVM I Princeton University COS 495 Instructor: Yingyu Liang

2 Review: machine learning basics

3 Math formulation Given training data x i, y i : 1 i n i.i.d. from distribution D Find y = f(x) H that minimizes L f = 1 σ n i=1 n l(f, x i, y i ) s.t. the expected loss is small L f = E x,y ~D [l(f, x, y)]

4 Machine learning Collect data and extract features Build model: choose hypothesis class H and loss function l Optimization: minimize the empirical loss

5 Loss function l 2 loss: linear regression Cross-entropy: logistic regression Hinge loss: Perceptron General principle: maximum likelihood estimation (MLE) l 2 loss: corresponds to Normal distribution logistic regression: corresponds to sigmoid conditional distribution

6 Optimization Linear regression: closed form solution Logistic regression: gradient descent Perceptron: stochastic gradient descent General principle: local improvement SGD: Perceptron; can also be applied to linear regression/logistic regression

7 Principle for hypothesis class? Yes, there exists a general principle (at least philosophically) Different names/faces/connections Occam s razor VC dimension theory Minimum description length Tradeoff between Bias and variance; uniform convergence The curse of dimensionality Running example: Support Vector Machine (SVM)

8 Motivation

9 Linear classification (w ) T x = 0 (w ) T x > 0 Class +1 w (w ) T x < 0 Class -1 Assume perfect separation between the two classes

10 Attempt Given training data Hypothesis y = sign(f w x ) = sign(w T x) y = +1 if w T x > 0 y = 1 if w T x < 0 x i, y i : 1 i n i.i.d. from distribution D Let s assume that we can optimize to find w

11 Multiple optimal solutions? Class +1 w 1 w 2 w 3 Class -1 Same on empirical loss; Different on test/expected loss

12 What about w 1? New test data Class +1 w 1 Class -1

13 What about w 3? New test data Class +1 w 3 Class -1

14 Most confident: w 2 New test data Class +1 w 2 Class -1

15 Intuition: margin large margin Class +1 w 2 Class -1

16 Margin

17 Margin Lemma 1: x has distance f w x w Proof: w is orthogonal to the hyperplane The unit direction is The projection of x is w w w w T to the hyperplane f w x = w T x = 0 x = f w(x) w 0 w w T x w w x

18 Margin: with bias Claim 1: w is orthogonal to the hyperplane f w,b x = w T x + b = 0 Proof: pick any x 1 and x 2 on the hyperplane w T x 1 + b = 0 w T x 2 + b = 0 So w T (x 1 x 2 ) = 0

19 Margin: with bias Claim 2: 0 has distance b w to the hyperplane wt x + b = 0 Proof: pick any x 1 the hyperplane Project x 1 to the unit direction w w T w w x 1 = b w since wt x 1 + b = 0 to get the distance

20 Margin: with bias Lemma 2: x has distance f w,b x b = 0 Proof: w Let x = x + r w, then r is the distance w Multiply both sides by w T and add b Left hand side: w T x + b = f w,b x Right hand side: w T x + r wt w w to the hyperplane f w,b x = w T x + + b = 0 + r w

21 The notation here is: y x = w T x + w 0 Figure from Pattern Recognition and Machine Learning, Bishop

22 Support Vector Machine (SVM)

23 SVM: objective Margin over all training data points: γ = min i f w,b x i w Since only want correct f w,b, and recall y i {+1, 1}, we have y i f w,b x i γ = min i w If f w,b incorrect on some x i, the margin is negative

24 SVM: objective Maximize margin over all training data points: max w,b γ = max w,b min i y i f w,b x i w = max w,b min i y i (w T x i + b) w A bit complicated

25 SVM: simplified objective Observation: when (w, b) scaled by a factor c, the margin unchanged y i (cw T x i + cb) cw = y i(w T x i + b) w Let s consider a fixed scale such that y i w T x i + b = 1 where x i is the point closest to the hyperplane

26 SVM: simplified objective Let s consider a fixed scale such that y i w T x i + b = 1 where x i is the point closet to the hyperplane Now we have for all data y i w T x i + b 1 and at least for one i the equality holds Then the margin is 1 w

27 SVM: simplified objective Optimization simplified to min w,b w y i w T x i + b 1, i How to find the optimum w?

28 SVM: principle for hypothesis class

29 Thought experiment Suppose pick an R, and suppose can decide if exists w satisfying 1 2 w 2 R y i w T x i + b 1, i Decrease R until cannot find w satisfying the inequalities

30 Thought experiment w is the best weight (i.e., satisfying the smallest R) w

31 Thought experiment w is the best weight (i.e., satisfying the smallest R) w

32 Thought experiment w is the best weight (i.e., satisfying the smallest R) w

33 Thought experiment w is the best weight (i.e., satisfying the smallest R) w

34 Thought experiment w is the best weight (i.e., satisfying the smallest R) w

35 Thought experiment To handle the difference between empirical and expected losses Choose large margin hypothesis (high confidence) Choose a small hypothesis class w Corresponds to the hypothesis class

36 Thought experiment Principle: use smallest hypothesis class still with a correct/good one Also true beyond SVM Also true for the case without perfect separation between the two classes Math formulation: VC-dim theory, etc. w Corresponds to the hypothesis class

37 Thought experiment Principle: use smallest hypothesis class still with a correct/good one Whatever you know about the ground truth, add it as constraint/regularizer w Corresponds to the hypothesis class

38 SVM: optimization Optimization (Quadratic Programming): min w,b w y i w T x i + b 1, i Solved by Lagrange multiplier method: L w, b, α = 1 2 w 2 where α is the Lagrange multiplier Details in next lecture i α i [y i w T x i + b 1]

39 Reading Review Lagrange multiplier method E.g. Section 5 in Andrew Ng s note on SVM posted on the course website: