Setting confidence intervals for bounded parameters

Abstract

Setting confidence bounds is an essential part of the reporting of experimental results. Current physics experiments are often done to measure nonnegative parameters that are small and may be zero and to search for small signals in the presence of backgrounds. These are examples of experiments which offer the possibility of yielding a result, recognized a priori to be relatively improbable, of a negative estimate for a quantity known to be positive. The classical Neyman procedure for setting confidence bounds in this situation is arguably unsatisfactory and several alternatives have been recently proposed. We compare methods for setting Gaussian and Poisson confidence intervals for cases in which the parameter to be estimated is bounded. These procedures lead to substantially different intervals when a relatively improbable observation implies a parameter estimate beyond the bound.