A simple alternative confidence interval for the difference between two proportions

The difference between two proportions is often the focus of interest in prospective comparative studies such as randomized controlled trials that have a binary outcome. Consequently, interval estimation for this parameter has received considerable attention in the literature. A hybrid procedure resulting from combining two sets of confidence limits for a single proportion as proposed by Newcombe has been previously recommended for this purpose because of its superior properties and relative simplicity. In this paper, we propose a simple alternative approach based on Fisher's z transformation. The results of an exact evaluation study show that this new procedure performs as well as Newcombe's procedure in terms of percent coverage and expected confidence interval width. Several examples are presented.