New Theoretical Frameworks for Machine Learning

1. New Theoretical Frameworks for Machine Learning Maria-Florina Balcan Thesis Proposal 05/15/2007

2. Thanks to My Committee Avrim Blum Manuel Blum Tom Mitchell Yishay Mansour Santosh Vempala

3. The Goal of the Thesis New Theoretical Frameworks for Modern Machine Learning Paradigms Connections between Machine Learning Theory and Algorithmic Game Theory

4. New Frameworks for Modern Learning Paradigms Modern Learning Paradigms Incorporating UnlabeledData in the Learning Process Kernel based Learning Qualitative gapbetween theory and practice Semi-supervised Learning Unified theoretical treatment is lacking Active Learning Our Contributions Our Contributions Semi-supervised learning A theory of learning with general similarity functions - a unified PAC framework Active Learning Extensions to clustering - new positive theoretical results With Avrim and Santosh

5. New Frameworks for Modern Learning Paradigms Modern Learning Paradigms Incorporating UnlabeledData in the Learning Process Kernel, Similarity based Learning and Clustering Qualitative gapbetween theory and practice Unified theoretical treatment is lacking Our Contributions Our Contributions Semi-supervised learning A theory of learning with general similarity functions - a unified PAC framework Active Learning Extensions to clustering - new positive theoretical results With Avrim and Santosh

6. Machine Learning Theory and Algorithmic Game Theory Brief Overview of Our Results Mechanism Design, ML, and Pricing Problems Generic Framework for reducing problems of incentive-compatible mechanism design to standard algorithmic questions. [Balcan-Blum-Hartline-Mansour, FOCS 2005, JCSS 2007] Approximation Algorithms for Item Pricing. [Balcan-Blum, EC 2006] • Revenue maximizationin comb. auctions with single-minded consumers

7. The Goal of the Thesis New Theoretical Frameworks for Modern Machine Learning Paradigms • Semi-Supervised and Active Learning • Similarity Based Learning and Clustering Connections between Machine Learning Theory and Algorithmic Game Theory • Use MLT techniques for designing and analyzing auctions in the context of Revenue Maximization

8. The Goal of the Thesis New Theoretical Frameworks for Modern Machine Learning Paradigms Incorporating UnlabeledData in the Learning Process Kernel, Similarity based learning and Clustering Semi-supervised learning (SSL) - Connections between kernels, margins and feature selection - An Augmented PAC model for SSL [Balcan-Blum, COLT 2005; book chapter, “Semi-Supervised Learning”, 2006] [Balcan-Blum-Vempala, MLJ 2006] - A general theory of learning with similarity functions Active Learning (AL) - Generic agnostic AL procedure [Balcan-Blum, ICML 2006] [Balcan-Beygelzimer-Langford, ICML 2006] - Extensions to Clustering - Margin based AL of linear separators [Balcan-Blum-Vempala, work in progress] [Balcan-Broder-Zhang, COLT 2007]

9. The Goal of the Thesis New Theoretical Frameworks for Modern Machine Learning Paradigms Incorporating UnlabeledData in the Learning Process Kernel, Similarity based learning and Clustering Semi-supervised learning (SSL) - Connections between kernels, margins and feature selection - An Augmented PAC model for SSL [Balcan-Blum, COLT 2005; book chapter, “Semi-Supervised Learning”, 2006] [Balcan-Blum-Vempala, MLJ 2006] - A general theory of learning with similarity functions Active Learning (AL) - Generic agnostic AL procedure [Balcan-Blum, ICML 2006] [Balcan-Beygelzimer-Langford, ICML 2006] - Extensions to Clustering - Margin based AL of linear separators [Balcan-Blum-Vempala, work in progress] [Balcan-Broder-Zhang, COLT 2007]

10. Part I, Incorporating Unlabeled Data in the Learning Process Semi-Supervised Learning A unified PAC-style framework [Balcan-Blum, COLT 2005; book chapter, “Semi-Supervised Learning”, 2006]

11. Standard Supervised Learning Setting • X – instance/feature space • S={(x, l)} - set of labeled examples • labeled examples - assumed to be drawn i.i.d. from some distr. D over X and labeled by some target concept c*2 C • labels 2{-1,1} - binary classification • Want to do optimization over S to find some hypothesis h, but we wanth to have small error over D. • err(h)=Prx 2 D(h(x)  c*(x)) • Classic models for learning from labeled data. • Statistical Learning Theory (Vapnik) • PAC (Valiant)

12. Standard Supervised Learning Setting Sample Complexity • E.g., Finite Hypothesis Spaces, Realizable Case • In PAC, can also talk about efficient algorithms.

13. Semi-Supervised Learning Hot topic in recent years in Machine Learning. • Several methods have been developed to try to use unlabeled data to improve performance, e.g.: • Transductive SVM[Joachims ’98] • Co-training[Blum & Mitchell ’98], [Balcan-Blum-Yang’04] • Graph-based methods[Blum & Chawla01], [ZGL03] Scattered Theoretical Results…

14. An Augmented PAC model for SSL [BB05] Extends PAC naturally to fit SSL. Can generically analyze: • When will unlabeled data help and by how much. • How muchdata should I expect to need to perform well. Key Insight Unlabeled data is useful if we have beliefs not only about the form of the target, but also about its relationship with the underlying distribution. Different algorithms are based on different assumptions about how data should behave. Challenge – how to capture many of the assumptions typically used.

15. _ + _ _ + + + _ + + _ _ SVM Transductive SVM Labeled data only Example of “typical” assumption: Margins The separator goes throughlowdensity regions of the space/large margin. • assume we are looking for linear separator • belief: should exist one withlargeseparation

16. Prof. Avrim Blum My Advisor Prof. Avrim Blum My Advisor x - Link info & Text info x2- Link info x1- Text info Another Example: Self-consistency Agreement between two parts : co-training [BM98]. - examples contain twosufficient sets of features, x = hx1, x2i - thebeliefis that the two parts of the example are consistent, i.e. 9 c1, c2 such that c1(x1)=c2(x2)=c*(x) For example, if we want to classify web pages: x = hx1, x2i

17. Problems thinking about SSL in the PAC model Su={xi} -unlabeledexamples drawn i.i.d. from D Sl={(xi, yi)} – labeled examples drawn i.i.d. from D and labeled by some target concept c*. PAC model talks of learning a class C under (known or unknown) distribution D. • Not clear what unlabeled data can do for you. • Doesn’t give you any info about which c 2 C is the target function. We extend the PAC model to capture these (and more) uses of unlabeled data. • Give aunified frameworkfor understanding when and why unlabeled data can help.

18. _ + + _ Proposed Model, Main Idea (1) Augment the notion of a concept classC with a notion of compatibilitybetween a concept and the data distribution. “learn C” becomes “learn (C,)” (i.e. learn class C under compatibility notion ) Express relationships that one hopes the target function and underlying distribution will possess. Idea: use unlabeled data & the belief that the target is compatible to reduce C down to just {the highly compatible functions in C}.

19. Proposed Model, Main Idea (2) Idea: use unlabeled data & our belief toreduce size(C) down to size(highly compatible functions in C) in our sample complexity bounds. Need to be able to analyze how much unlabeled data is needed to uniformly estimate compatibilities well. Require that the degree of compatibility be something that can be estimated from a finite sample. • Require  to be an expectation over individual examples: • (h,D)=Ex2 D[(h, x)]compatibility of h with D, (h,x)2 [0,1] • errunl(h)=1-(h, D) incompatibility of h with D (unlabeled error rate of h)

20. _ + Highly compatible + _ Margins, Compatibility Margins: belief is that should exist a large margin separator. Incompatibility of h and D (unlabeled error rate of h) – the probability mass within distance  of h. Can be written as an expectation over individual examples(h,D)=Ex 2 D[(h,x)] where:(h,x)=0 if dist(x,h) ·(h,x)=1 if dist(x,h) ¸

21. _ + Highly compatible + _ Margins, Compatibility Margins: belief is that should exist a large margin separator. If do not want to commit to in advance, define (h,x) to be a smooth function of dist(x,h), e.g.: Illegal notion of compatibility: the largest s.t. D has probability mass exactly zero within distance  of h.

22. Co-Training, Compatibility Co-training: examples come as pairs hx1, x2i and the goal is to learn a pair of functionshh1,h2i. Hope is that the two parts of the example are consistent. Legal (and natural)notion of compatibility: - the compatibility of hh1,h2iand D: - can be written as an expectation over examples:

23. Types of Results in the [BB05] Model As in PAC, can discuss algorithmic and sample complexity issues. Sample Complexity issues that we can address: • How much unlabeled data we need: • depends both on the complexity of C and the on the • complexity of our notion of compatibility. - Ability of unlabeled data to reduce # of labeled examples needed: • compatibility of the target • (various) measures of the helpfulness of the distribution • Give both uniform convergence bounds and epsilon-cover based bounds.

24. Examples of results:Sample Complexity, Uniform Convergence Bounds Finite Hypothesis Spaces, Doubly Realizable Case ALG: pick a compatible concept that agrees with the labeled sample. CD,() = {h 2 C :errunl(h) ·} Bound the # of labeled examples as a measure of the helpfulness of D with respect to  • helpful D is one in which CD, () is small

25. + _ Highly compatible + _ Examples of results:Sample Complexity, Uniform Convergence Bounds Finite Hypothesis Spaces, Doubly Realizable Case ALG: pick a compatible concept that agrees with the labeled sample. CD,() = {h 2 C :errunl(h) ·}

26. + _ Highly compatible + _ Sample Complexity Subtleties Uniform Convergence Bounds Depends both on the complexity of C and on the complexity of  Distr. dependent measure of complexity -Cover boundsmuch better than Uniform Convergence bounds. • For algorithms that behave in a specific way: • first use the unlabeled data to choose a representative set of compatible hypotheses • then use the labeled sample to choose among these

27. Sample Complexity Implications of Our Analysis Ways in which unlabeled data can help • If c* is highly compatible and have enough unlabeled data, then can reduce the search space (from C down to just those h 2 C whose estimated unlabeled error rate is low). • By providing an estimate of D, unlabeled data can allow a more refined distribution-specific notion of hypothesis space size (e.g. the size of the smallest -cover). Subsequent Work, E.g.: P. Bartlett, D. Rosenberg, AISTATS 2007 J. Shawe-Taylor et al., Neurocomputing 2007

28. Efficient Co-training of linear separators • Assume independence given the label • both points from D+ or from D-. • [Blum & Mitchell] show can co-train (in polynomial time) if have enoughlabeled data to produce a weakly-usefulhypothesis to begin with. • [BB05] shows we can learn (in polynomial time) with only a single labeled example. • Key point: independence given the label implies that the functions with low errunl rate are: • close to c* • close to : c* • close to the all positive function • close to the all negative function Idea: use unlabeled data to generate poly # of candidate hyps s.t. at least one is weakly-useful (uses Outlier Removal Lemma). Plug into [BM98].

29. Modern Learning Paradigms: Our Contributions Modern Learning Paradigms Incorporating Unlabeled Data in the Learning Process Kernel, Similarity based learning and Clustering Semi-supervised learning (SSL) - Connections between kernels, margins and feature selection - An Augmented PAC model for SSL [Balcan-Blum-Vempala, MLJ 2006] [Balcan-Blum, COLT 2005] [Balcan-Blum, book chapter, “Semi-Supervised Learning”, 2006] - A general theory of learning with similarity functions Active Learning (AL) [Balcan-Blum, ICML 2006] - Generic agnostic AL procedure - Extensions to Clustering [Balcan-Beygelzimer-Langford, ICML 2006] [Balcan-Blum-Vempala, work in progress] - Margin based AL of linear separators [Balcan-Broder-Zhang, COLT 2007]

30. Modern Learning Paradigms: Our Contributions Modern Learning Paradigms Incorporating Unlabeled Data in the Learning Process Kernel, Similarity based learning and Clustering Semi-supervised learning (SSL) - Connections between kernels, margins and feature selection - An Augmented PAC model for SSL [Balcan-Blum-Vempala, MLJ 2006] [Balcan-Blum, COLT 2005] [Balcan-Blum, book chapter, “Semi-Supervised Learning”, 2006] - A general theory of learning with similarity functions Active Learning (AL) [Balcan-Blum, ICML 2006] - Generic agnostic AL procedure - Extensions to Clustering [Balcan-Beygelzimer-Langford, ICML 2006] [Balcan-Blum-Vempala, work in progress] - Margin based AL of linear separators [Balcan-Broder-Zhang, COLT 2007]

31. Part II, Similarity Functions for Learning [Balcan-Blum, ICML 2006] Extensions to Clustering (With Avrim and Santosh, work in progress)

32. Kernels and Similarity Functions Kernels have become a powerful tool in ML. • Useful in practice for dealing with many different kinds of data. • Elegant theory about what makes a given kernel good for a given learning problem. Our Work: analyze more general similarity functions. • In the process we describe ways of constructing good data dependent kernels.

33.  (x) 1 w Kernels • A kernel K is a pairwise similarity function s.t. 9 an implicit mapping  s.t. K(x,y)=(x) ¢(y). • Point is: many learning algorithms can be written so only interact with data via dot-products. • If replace x¢y with K(x,y), it acts implicitly as if data was in higher-dimensional -space. • If data is linearly separable by large margin in -space, don’t have to pay in terms of data or comp time. If margin  in -space, only need 1/2 examples to learn well.

34. General Similarity Functions We provide:characterization ofgood similarity functionsfor a learning problem that: 1) Talks in terms of natural direct properties: • no implicit high-dimensional spaces • no requirement of positive-semidefiniteness 2) If K satisfies these properties for our given problem, then has implications to learning. 3) Is broad: includes usual notion of “good kernel”. (induces a large margin separator in -space)

35. - B C - A + A First Attempt: Definition satisfying properties (1) and (2) Let P be a distribution over labeled examples (x, l(x)) • K:(x,y) ! [-1,1] is an (,)-good similarity for P if at leasta 1-probability mass of x satisfy: Ey~P[K(x,y)|l(y)=l(x)] ¸ Ey~P[K(x,y)|l(y)l(x)]+ • Suppose that positives have K(x,y) ¸ 0.2, negatives have K(x,y) ¸ 0.2, but for a positive and a negative K(x,y) are uniform random in [-1,1]. Note: this might not be a legal kernel.

36. A First Attempt: Definition satisfying properties (1) and (2). How to use it? • K:(x,y) ! [-1,1] is an(,)-good similarityfor P if at leasta 1-probability mass of x satisfy: Ey~P[K(x,y)|l(y)=l(x)] ¸ Ey~P[K(x,y)|l(y)l(x)]+ Algorithm • Draw S+ of O((1/2) ln(1/2)) positive examples. • Draw S- of O((1/2) ln(1/2)) negative examples. • Classify x based on which gives better score.

37. A First Attempt: How to use it? • K:(x,y) ! [-1,1] is an(,)-good similarityfor P if at leasta 1-probability mass ofx satisfy: Ey~P[K(x,y)|l(y)=l(x)] ¸ Ey~P[K(x,y)|l(y)l(x)]+ Algorithm • Draw S+ of O((1/2) ln(1/2)) positive examples. • Draw S- of O((1/2) ln(1/2)) negative examples. • Classify x based on which gives better score. Guarantee: with probability ¸1-, error · + . Proof • Hoeffding: for any given “goodx”, probability of error w.r.t. x (over draw of S+, S-) at most 2. • By Markov, at most  chance that the error rate over GOOD is more than . So overall error rate · + .

38. more similar to negs than to typical pos + + + + + + - - - - - - A First Attempt: Not Broad Enough • K:(x,y) ! [-1,1] is an(,)-good similarityfor P if at leasta 1-probability mass of x satisfy: Ey~P[K(x,y)|l(y)=l(x)] ¸ Ey~P[K(x,y)|l(y)l(x)]+ • K(x,y)=x ¢ y has large margin separator but doesn’t satisfy our definition.

39. A First Attempt: Not Broad Enough • K:(x,y) ! [-1,1] is an(,)-good similarityfor P if at leasta 1-probability mass of x satisfy: Ey~P[K(x,y)|l(y)=l(x)] ¸ Ey~P[K(x,y)|l(y)l(x)]+ R + + + + + + - - - - - - Idea: would work if we didn’t pick y’s from top-left. Broaden to say:OK if 9 non-negligable region R s.t. most x are on average more similar to y2R of same label than to y2 R of other label.

40. Broader/Main Definition • K:(x,y) ! [-1,1] is an(,)-good similarityfor P if exists a weighting functionw(y) 2 [0,1]at leasta 1-probability mass of x satisfy: Ey~P[w(y)K(x,y)|l(y)=l(x)] ¸ Ey~P[w(y)K(x,y)|l(y)l(x)]+

41. Main Definition, How to Use It • K:(x,y) ! [-1,1] is an(,)-good similarityfor P if exists a weighting functionw(y) 2 [0,1] at leasta 1-probability mass of x satisfy: Ey~P[w(y)K(x,y)|l(y)=l(x)] ¸ Ey~P[w(y)K(x,y)|l(y)l(x)]+ Algorithm • Draw S+={y1, , yd}, S-={z1, , zd}, d=O((1/2) ln(1/2)). • Use to “triangulate” data: F(x) = [K(x,y1), …,K(x,yd), K(x,zd),…,K(x,zd)]. • Take a new set of labeled examples, project to this space, and run your favorite alg for learning lin. separators. Point is: with probability ¸ 1-, exists linear separator of error · + at margin /4. (w = [w(y1), …,w(yd),-w(zd),…,-w(zd)])

42. Main Definition, Implications Algorithm • Draw S+={y1, , yd}, S-={z1, , zd}, d=O((1/2) ln(1/2)). • Use to “triangulate” data: F(x) = [K(x,y1), …,K(x,yd), K(x,zd),…,K(x,zd)]. Guarantee: with prob. ¸ 1-, exists linear separator of error · + at margin /4. legal kernel Implications K arbitrary sim. function (,)-goodsim. function (+,/4)-goodkernelfunction

43. Good Kernels are Good Similarity Functions Main Definition: K:(x,y) ! [-1,1] is an(,)-good similarityfor P if exists a weighting functionw(y) 2 [0,1] at leasta 1-probability mass of x satisfy: Ey~P[w(y)K(x,y)|l(y)=l(x)] ¸ Ey~P[w(y)K(x,y)|l(y)l(x)]+ Theorem • An (,)-good kernel is an (’,’)-good similarity function under main definition. Our proofs incurred some penalty: ’ =  + extra, ’ = 3extra. Nati Srebro (COLT 2007) has improved the bounds.

44. Sample complexity is roughly Learning with Multiple Similarity Functions • Let K1, …, Kr be similarity functions s. t. some (unknown) convex combination of them is (,)-good. Algorithm • Draw S+={y1, , yd}, S-={z1, , zd}, d=O((1/2) ln(1/2)). • Use to “triangulate” data: F(x) = [K1(x,y1), …,Kr(x,yd), K1(x,zd),…,Kr(x,zd)]. Guarantee: The induced distribution F(P) in R2dr has a separator of error · +  at margin at least

45. Implications • Theory that provides a formal way of understanding kernels as similarity functions. • Algorithms work for sim. fns that aren’t necessarily PSD. • Suggests natural approach for using similarity functions to augment feature vector in “anytime” way. • E.g., features for document can be list of words in it, plus similarity to a few “landmark” documents. • Formal justification for “Feature Generation for Text Categorization using World Knowledge”, GM’05 Mugizi has proposed on this

46. Clustering via Similarity Functions (Work in Progress, with Avrim and Santosh)

47. What if only unlabeled examples available? Consider the following setting: • Given data set S of n objects. • There is some (unknown) “ground truth” clustering. Each x has true label l(x) in {1,…,t}. • Goal: produce hypothesis h of low error up to isomorphism of label names. [documents, web pages] [topic] People have traditionally considered mixture models here. Can we say something in our setting?

48. What if only unlabeled examples available? • Suppose our similarity function satisfies the stronger condition: • Ground truth is “stable” in that • Then, can construct a tree (hierarchical clustering) such that the correct clustering is some pruning of this tree. For all clusters C, C’, for all A in C, A’ in C’: A and A’ are not both more attracted to each other than to their own clusters. K(x,y) is attraction between x and y

49. What if only unlabeled examples available? • Suppose our similarity function satisfies the stronger condition: • Ground truth is “stable” in that For all clusters C, C’, for all A in C, A’ in C’: A and A’ are not both more attracted to each other than to their own clusters. K(x,y) is attraction between x and y fashion sports volleyball Dolce & Gabbana soccer Cocco Chanel gymnastics

50. Main point • Exploring the question: what are minimal conditions on a similarity function that allow it to be useful for clustering? • Have considered two relaxations of the Clustering objective: • List Clustering -- small number of candidate clusterings. • Hierarchical clustering -- output a tree such that right answer is some pruning of it. • Allow for right answer to be identified with a little bit of additional feedback.