We introduce a new approach for designing computationally efficient learning algorithms that are tolerant to noise. We demonstrate the effectiveness of our approach by designing algorithms with improved noise tolerance guarantees for learning linear separators. We consider the malicious noise model of Valiant and the adversarial label noise model of Kearns, Schapire, and Sellie. For malicious noise, where the adversary can corrupt an @math of fraction both the label part and the feature part, we provide a polynomial-time algorithm for learning linear separators in @math under the uniform distribution with near information-theoretic optimal noise tolerance of @math . We also get similar improvements for the adversarial label noise model. We obtain similar results for more general classes of distributions including isotropic log-concave distributions. In addition, our algorithms achieve a label complexity whose dependence on the error parameter @math is exponentially better than that of any passive algorithm. This provides the first polynomial-time active learning algorithm for learning linear separators in the presence of adversarial label noise, as well as the first analysis of active learning under the malicious noise model.
@cite_27 considered noise-tolerant learning of halfspaces under a more idealized noise model, known as the random noise model, in which the label of each example is flipped with a certain probability, independently of the feature vector. Some other, less closely related, work on efficient noise-tolerant learning of halfspaces includes @cite_0 @cite_27 @cite_37 @cite_9 @cite_11 @cite_6 @cite_19 @cite_42 .
@cite_37 @cite_9 @cite_42 @cite_6 @cite_0 @cite_19 @cite_27 @cite_11
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We address well-studied problems concerning the learnability of parities and halfspaces in the presence of classification noise. Learning of parities under the uniform distribution with random classification noise, also called the noisy parity problem is a famous open problem in computational learning. We reduce a number of basic problems regarding learning under the uniform distribution to learning of noisy parities. We show that under the uniform distribution, learning parities with adversarial classification noise reduces to learning parities with random classification noise. Together with the parity learning algorithm of [5], this gives the first nontrivial algorithm for learning parities with adversarial noise. We show that learning of DNF expressions reduces to learning noisy parities of just logarithmic number of variables. We show that learning of k-juntas reduces to learning noisy parities of k variables. These reductions work even in the presence of random classification noise in the original DNF or junta. We then consider the problem of learning halfspaces over Q ^n with adversarial noise or finding a halfspace that maximizes the agreement rate with a given set of examples. We prove an essentially optimal hardness factor of 2- , improving the factor of 85 84 - due to Bshouty and Burroughs [8]. Finally, we show that majorities of halfspaces are hard to PAC-learn using any representation, based on the cryptographic assumption underlying the Ajtai-Dwork cryptosystem. Learning an unknown halfspace (also called a perceptron) from labeled examples is one of the classic problems in machine learning. In the noise-free case, when a halfspace consistent with all the training examples exists, the problem can be solved in polynomial time using linear programming. However, under the promise that a halfspace consistent with a fraction @math of the examples exists (for some small constant @math ), it was not known how to efficiently find a halfspace that is correct on even 51 of the examples. Nor was a hardness result that ruled out getting agreement on more than 99.9 of the examples known. In this work, we close this gap in our understanding and prove that even a tiny amount of worst-case noise makes the problem of learning halfspaces intractable in a strong sense. Specifically, for arbitrary @math , we prove that given a set of examples-label pairs from the hypercube, a fraction @math of which can be explained by a halfspace, it is NP-hard to find a halfspace that correctly labels a fraction @math of the examples. The hardness result is tight since it is trivial to get agreement on @math the examples. In learning theory parlance, we prove that weak proper agnostic learning of halfspaces is hard. This settles a question that was raised by , in their work on learning halfspaces in the presence of random classification noise [Algorithmica, 22 (1998), pp. 35-52], and raised by authors of some more recent works as well. Along the way, we also obtain a strong hardness result for another basic computational problem: solving a linear system over the rationals. In this paper, we study the problem of learning in the presence of classification noise in the probabilistic learning model of Valiant and its variants. In order to identify the class of “robust” learning algorithms in the most general way, we formalize a new but related model of learning from statistical queries . Intuitively, in this model a learning algorithm is forbidden to examine individual examples of the unknown target function, but is given acess to an oracle providing estimates of probabilities over the sample space of random examples. One of our main results shows that any class of functions learnable from statistical queries is in fact learnable with classification noise in Valiant's model, with a noise rate approaching the information-theoretic barrier of 1 2. We then demonstrate the generality of the statistical query model, showing that practically every class learnable in Valiant's model and its variants can also be learned in the new model (and thus can be learned in the presence of noise). A notable exception to this statement is the class of parity functions, which we prove is not learnable from statistical queries, and for which no noise-tolerant algorithm is known. We give a computationally efficient algorithm that learns (under distributional assumptions) a halfspace in the difficult agnostic framework of Kearns, Schapire, and Sellie [Mach. Learn., 17 (1994), pp. 115-141], where a learner is given access to a distribution on labelled examples but where the labelling may be arbitrary (similar to malicious noise). It constructs a hypothesis whose error rate on future examples is within an additive @math of the optimal halfspace, in time poly @math for any constant @math , for the uniform distribution over @math or unit sphere in @math as well as any log-concave distribution in @math . It also agnostically learns Boolean disjunctions in time @math with respect to any distribution. Our algorithm, which performs @math polynomial regression, is a natural noise-tolerant arbitrary-distribution generalization of the well-known “low-degree” Fourier algorithm of Linial, Mansour, and Nisan. We observe that significant improvements on the running time of our algorithm would yield the fastest known algorithm for learning parity with noise, a challenging open problem in computational learning theory.