PAC-learning, VC Dimension and Margin-based Bounds
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1 More details: General: Example of more complex bounds: PAC-learning, VC Dimension and Margin-based Bounds Machine Learning 10701/15781 Carlos Guestrin Carnegie Mellon University February 28 th, 2005
2 Review: Generalization error in finite hypothesis spaces [Haussler 88] Theorem: Hypothesis space H finite, dataset D with m i.i.d. samples, 0 < ε < 1 : for any learned hypothesis h that is consistent on the training data: Even if h makes zero errors in training data, may make errors in test
3 Using a PAC bound Typically, 2 use cases: 1: Pick ε and δ, give you m 2: Pick m and δ, give you ε
4 Limitations of Haussler 88 bound Consistent classifier Size of hypothesis space
5 What if our classifier does not have zero error on the training data? A learner with zero training errors may make mistakes in test set A learner with error D (h) in training set, may make even more mistakes in test set
6 Simpler question: What s the expected error of a hypothesis? The error of a hypothesis is like estimating the parameter of a coin! Chernoff bound: for m i.d.d. coin flips, x 1,,x m, where x i {0,1}. For 0<ε<1:
7 Using Chernoff bound to estimate error of a single hypothesis
8 But we are comparing many hypothesis: Union bound For each hypothesis h i : What if I am comparing two hypothesis, h 1 and h 2?
9 Generalization bound for H hypothesis Theorem: Hypothesis space H finite, dataset D with m i.i.d. samples, 0 < ε < 1 : for any learned hypothesis h:
10 PAC bound and Bias-Variance tradeoff or, after moving some terms around, with probability at least 1-δ: Important: PAC bound holds for all h, but doesn t guarantee that algorithm finds best h!!!
11 What about the size of the hypothesis space? How large is the hypothesis space?
12 Boolean formulas with n binary features
13 Number of decision trees of depth k Recursive solution Given n attributes H k = Number of decision trees of depth k H 0 =2 H k+1 = (#choices of root attribute) * (# possible left subtrees) * (# possible right subtrees) = n * H k * H k Write L k = log 2 H k L 0 = 1 L k+1 = log 2 n + 2L k So L k = (2 k -1)(1+log 2 n) +1
14 PAC bound for decision trees of depth k Bad!!! Number of points is exponential in depth! But, for m data points, decision tree can t get too big Number of leaves never more than number data points
15 Number of decision trees with k leaves H k = Number of decision trees with k leaves H 0 =2 Loose bound: Reminder:
16 PAC bound for decision trees with k leaves Bias-Variance revisited
17 What did we learn from decision trees? Bias-Variance tradeoff formalized Moral of the story: Complexity of learning not measured in terms of size hypothesis space, but in maximum number of points that allows consistent classification Complexity m no bias, lots of variance Lower than m some bias, less variance
18 What about continuous hypothesis spaces? Continuous hypothesis space: H = Infinite variance??? As with decision trees, only care about the maximum number of points that can be classified exactly!
19 How many points can a linear boundary classify exactly? (1-D)
20 How many points can a linear boundary classify exactly? (2-D)
21 How many points can a linear boundary classify exactly? (d-d)
22 PAC bound using VC dimension Number of training points that can be classified exactly is VC dimension!!! Measures relevant size of hypothesis space, as with decision trees with k leaves
23 Shattering a set of points
24 VC dimension
25 Examples of VC dimension Linear classifiers: VC(H) = d+1, for d features plus constant term b Neural networks VC(H) = #parameters Local minima means NNs will probably not find best parameters 1-Nearest neighbor?
26 PAC bound for SVMs SVMs use a linear classifier For d features, VC(H) = d+1:
27 VC dimension and SVMs: Problems!!! Doesn t take margin into account What about kernels? Polynomials: num. features grows really fast = Bad bound n input features p degree of polynomial Gaussian kernels can classify any set of points exactly
28 Margin-based VC dimension H: Class of linear classifiers: w.φ(x) (b=0) Canonical form: min j w.φ(x j ) = 1 VC(H) = R 2 w.w Doesn t depend on number of features!!! R 2 = max j Φ(x j ).Φ(x j ) magnitude of data R 2 is bounded even for Gaussian kernels bounded VC dimension Large margin, low w.w, low VC dimension Very cool!
29 Applying margin VC to SVMs? VC(H) = R 2 w.w R 2 = max j Φ(x j ).Φ(x j ) magnitude of data, doesn t depend on choice of w SVMs minimize w.w SVMs minimize VC dimension to get best bound? Not quite right: Bound assumes VC dimension chosen before looking at data Would require union bound over infinite number of possible VC dimensions But, it can be fixed!
30 Structural risk minimization theorem For a family of hyperplanes with margin γ>0 w.w 1 SVMs maximize margin γ + hinge loss Optimize tradeoff training error (bias) versus margin γ (variance)
31 Reality check Bounds are loose ε d=2000 d=200 d=20 d=2 m (in 10 5 ) Bound can be very loose, why should you care? There are tighter, albeit more complicated, bounds Bounds gives us formal guarantees that empirical studies can t provide Bounds give us intuition about complexity of problems and convergence rate of algorithms
32 What you need to know Finite hypothesis space Derive results Counting number of hypothesis Mistakes on Training data Complexity of the classifier depends on number of points that can be classified exactly Finite case decision trees Infinite case VC dimension Bias-Variance tradeoff in learning theory Margin-based bound for SVM Remember: will your algorithm find best classifier?
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