## Complexities of convex combinations and bounding the generalization error in classification.(English)Zbl 1080.62045

From the paper: Since the invention of enemble classification methods (such as boosting), the convex hull $$\text{conv}({\mathcal H})$$ of a given base function class $${\mathcal H}$$ has become an important object of study in the machine learning literature. The reason is that the ensemble algorithms typically output classifiers that are convex combinations of simple classifiers selected by the algorithm from the base class $${\mathcal H}$$, and, because of this, measuring the complexity of the whole convex hull as well as of its subsets becomes very important in the analysis of the generalization error of ensemble classifiers. Another important feature of boosting and many other ensemble methods is that they belong to the class of so-called large margin methods, that is, they are based on optimization of the empirical risk with respect to various loss functions that penalize not only for a misclassification (a negative classification margin), but also for a correct classification with too small positive margin. Thus, the very nature of these methods is to produce classifiers that tend to have rather large positive classification margins on the training data. Finding such classifiers becomes possible since the algorithms search for them in rather huge function classes (such as convex hulls of typical VC-classes used in classification).
We introduce and study several measures of complexity of functions from the convex hull of a given base class. These complexity measures take into account the sparsity of the weights of a convex combination as well as certain clustering properties of the base functions involved in it. We prove new upper confidence bounds on the generalization error of ensemble (voting) classification algorithms that utilize the new complexity measures along with the empirical distributions of classification margins, providing a better explanation of generalization performance of large margin classification methods.

### MSC:

 62H30 Classification and discrimination; cluster analysis (statistical aspects) 65Y20 Complexity and performance of numerical algorithms 68T99 Artificial intelligence 62G05 Nonparametric estimation 62G20 Asymptotic properties of nonparametric inference 60F15 Strong limit theorems 68Q32 Computational learning theory 68Q25 Analysis of algorithms and problem complexity
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