Approximations of marginal tail probabilities for a class of smooth functions with applications to Bayesian and conditional inference

Abstract

SUMMARY This paper presents an asymptotic approximation of marginal tail probabilities for a real-valued function of a random vector, where the function has continuous gradient that does not vanish at the mode of the joint density of the random vector. This approximation has error O(n −3/2 ) and improves upon a related standard normal approximation which has error O(n −½ ). Derivation involves the application of a tail probability formula given by DiCiccio, Field & Fraser (1990) to an approximation of a marginal density derived by Tierney, Kass & Kadane (1989). The approximation can be applied for Bayesian and conditional inference as well as for approximating sampling distributions, and the accuracy of the approximation is illustrated through several numerical examples related to such applications. In the context of conditional inference, we develop refinements of the standard normal approximation to the distribution of two different signed root likelihood ratio statistics for a component of the natural parameter in exponential families.