An Application of an Urn Model to the Design of Sequential Controlled Clinical Trials

Abstract

Abstract In the comparison of K treatments, assume that patients appear singly and must be treated immediately. Suppose that patients having the same combination of prognostic factor levels are grouped into the same stratum. If the number of different strata is small, we treat each stratum as a separate independent subtrial. A treatment assignment rule is proposed that forces a small subtrial to be balanced, but tends toward the complete randomization scheme as the size of the subtrial increases. When the number of strata is large, we propose an overall assignment rule which can achieve a degree of treatment balance simultaneously across all prognostic factors.