Stochastically optimal bootstrap sample size for shrinkage-type statistics

In nonregular problems where the conventional n out of n bootstrap is inconsistent, the m out of n bootstrap provides a useful remedy to restore consistency. Conventionally, optimal choice of the bootstrap sample size m is taken to be the minimiser of a frequentist error measure, estimation of which has posed a major difficulty hindering practical application of the m out of n bootstrap method. Relatively little attention has been paid to a stronger, stochastic, version of the optimal bootstrap sample size, defined as the minimiser of an error measure calculated directly from the observed sample. Motivated by this stronger notion of optimality, we develop procedures for calculating the stochastically optimal value of m. Our procedures are shown to work under special forms of Edgeworth-type expansions which are typically satisfied by statistics of the shrinkage type. Theoretical and empirical properties of our methods are illustrated with three examples, namely the James-Stein estimator, the ridge regression estimator and the post-model-selection regression estimator.