Extending the scope of the small-ball method
Volume 256 / 2021
Abstract
The small-ball method was introduced as a way of obtaining a high probability, isomorphic lower bound on the quadratic empirical process, under weak assumptions on the indexing class. The key assumption was that class members satisfy a uniform small-ball estimate: that ${\rm Pr}(|f| \geq \kappa \|f\|_{L_2}) \geq \delta $ for given constants $\kappa $ and $\delta $.
Here we extend the small-ball method and obtain a high probability, almost-isometric (rather than isomorphic) lower bound on the quadratic empirical process. The scope of the result is considerably wider than the small-ball method: there is no need for class members to satisfy a uniform small-ball condition, and moreover, motivated by the notion of tournament learning procedures, the result is stable under a “majority vote”.
