Predictive sets with approximate frequentist and Bayesian validity for arbitrary priors

With a view to predicting a scalar-valued future observation on the basis of past observations, we explore predictive sets having frequentist as well as Bayesian validity for arbitrary priors in a higher-order asymptotic sense. It is found that a connection with locally unbiased tests is useful for this purpose. Illustrative examples are given. Computation and simulation studies lend support to our asymptotic results in finite samples. The issue of expected lengths of our predictive sets is also discussed.