Randomized Block Designs

Experimental criminologists pay close attention to baseline equality between the treatment and control groups. When the study groups are not similar to one another, this introduces biases and confusion in the interpretation of the treatment effect. Often simple random assignment procedures are used to create similar groups, where the entire sample is divided in half into treatment and control conditions. However, research shows that this simple process does not always work. By chance alone, one group can be larger and have more of a particular type of participants or type of traits. Such instances make it difficult to compare “like with like,” which is one of the cornerstones of randomized controlled trials. Instead, experimentalists and statisticians advise using randomized block designs, in which cases are randomly assigned within homogeneous blocks of participants, rather than from the overall sample. The variance within the blocks is minimized, which decreases the experimental "noise' and increases the statistical power of the test Randomized block designs can also be used to "force" size equality in order to create exactly the same group sizes, something which cannot be done with simple random assignment. Below, instances in which it is optimal to use randomized block experiments are discussed, as well as how to design them, and the obstacles to avoid when using these designs. The general type of randomized block designs is introduced along with two special cases: permuted randomized block designs and matched pairs, which are useful in small-scale studies where group-size equality is particularly important and when unmasking of the random allocation sequence does not create a source of concern.