Randomized Block Designs

Simple random allocation designs in RCTs cannot always guarantee balance in terms of variance, between-group size, or covariant effects. This is particularly the case in smaller trials of a few hundred units or less. These imbalances pose threats to the power of statistical tests, as well as to the precision of treatment estimates. By sacrificing complete randomization in the allocation of treatment(s) of experimental and control units, randomized block designs (RBD) can decrease such threats. Specifically, RBDs, where units are assigned to conditions within homogenized blocks based on a grouping criterion, are commonly employed in other disciplines. We therefore discuss these designs and their advantages for experimental criminology, whose experience with RBDs is fairly limited. Four types of RBDs are presented: the complete blocked randomization-, with and without an interaction term between the treatment and blocking factors, the balanced incomplete block randomization-, and the permuted-blocks randomization-designs. Each is a better fit for certain conditions that arise from the type of data analyzed. We discuss these designs, by showing why, as well as how, each can be implemented in criminology.