Center for Effective Global Action

In many randomized trials, subjects enter the sample sequentially. Because thecovariates for all units are not known in advance, standard methods of stratificationdo not apply. We describe and assess the method of DA-optimal sequential allocation(Atkinson, 1982) for balancing stratification covariates across treatment arms. Weprovide simulation evidence that the method can provide substantial improvementsin precision over commonly-employed alternatives. We also describe our experience implementing the method in a field trial of a clean water and handwashing intervention in Dhaka, Bangladesh, the first time the method has been used. We provide advice and software for future researchers.