It is the goal of this paper to extend the Empirical Risk Minimization (ERM) paradigm, from a practical perspective, to the situation where a natural estimate of the risk is of the form of a K-sample U-statistics, as it is the case in the K-partite ranking problem for instance. Indeed, the numerical computation of the empirical risk is hardly feasible if not infeasible, even for moderate samples sizes. Precisely, it involves averaging O(n d1+...+dK ) terms, when considering a U-statistic of degrees (d1, . . . , dK) based on samples of sizes proportional to n. We propose here to consider a drastically simpler Monte-Carlo version of the empirical risk based on O(n) terms solely, which can be viewed as an incomplete generalized U-statistic, and prove that, remarkably, the approximation stage does not damage the ERM procedure and yields a learning rate of order OP(1/ √ n). Beyond a theoretical analysis guaranteeing the validity of this approach, numerical experiments are displayed for illustrative purpose.
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