On agnostic boosting and parity learning

The motivating problem is agnostically learning parity functions, i.e., parity with arbitrary or adversarial noise. Specifically, given random labeled examples from an arbitrary distribution, we would like to produce an hypothesis whose accuracy nearly matches the accuracy of the best parity function. Our algorithm runs in time 2̊ (n/log n), which matches the best known for the easier cases of learning parities with random classification noise (Blum et al, 2003) and for agnostically learning parities over the uniform distribution on inputs (Feklman et al, 2006). Our approach is as follows. We give an agnostic boosting theorem that is capable of nearly achieving optimal accuracy, improving upon earlier studies (starting with Ben David et al, 2001). To achieve this, we circumvent previous lower bounds by altering the boosting model. We then show that the (random noise) parity learning algorithm of Blum et al (2000) fits our new model of agnostic weak learner. Our agnostic boosting framework is completely general and may be applied to other agnostic learning problems. Hence, it also sheds light on the actual difficulty of agnostic learning by showing that full agnostic boosting is indeed possible.