Optimal learners for multiclass problems
Proceedings of The 27th Conference on Learning Theory, PMLR 35:287-316, 2014.
The fundamental theorem of statistical learning states that for \emphbinary classification problems, any Empirical Risk Minimization (ERM) learning rule has close to optimal sample complexity. In this paper we seek for a generic optimal learner for \emphmulticlass prediction. We start by proving a surprising result: a generic optimal multiclass learner must be \emphimproper, namely, it must have the ability to output hypotheses which do not belong to the hypothesis class, even though it knows that all the labels are generated by some hypothesis from the class. In particular, no ERM learner is optimal. This brings back the fundamental question of “how to learn”? We give a complete answer to this question by giving a new analysis of the one-inclusion multiclass learner of Rubinstein et el (2006) showing that its sample complexity is essentially optimal. Then, we turn to study the popular hypothesis class of generalized linear classifiers. We derive optimal learners that, unlike the one-inclusion algorithm, are computationally efficient. Furthermore, we show that the sample complexity of these learners is better than the sample complexity of the ERM rule, thus settling in negative an open question due to Collins (2005)