APA Dictionary of Psychology

a type of research study in which participants are divided into relatively homogeneous subsets (blocks) from which they are assigned to the experimental or treatment conditions. For example, in a simple block design to evaluate the efficacy of several antidepressants, participants with similar pretest depression scores might be grouped into homogeneous blocks and then assigned to receive different medications. The purpose of a block design is to ensure that a characteristic of the study participants that is related to the target outcome (i.e., a covariate) is distributed equally across treatment conditions. See complete block design; incomplete block design; randomized block design.