Why are experimental units grouped into blocks in a randomized block design?

In a randomized block design, experimental units are grouped into blocks to ensure that comparisons between treatments are made within groups of homogeneous units. This technique is essential for increasing the precision of an experiment by controlling for external factors that might otherwise influence the response variable.1

By organizing units into blocks—such as grouping automobiles by manufacturer—researchers prevent extraneous variables from confounding the results. For example, if one manufacturer produces inherently more efficient engines, a completely randomized design might accidentally assign a specific treatment to more of those vehicles, leading to biased conclusions. Blocking ensures that each treatment is tested across the different categories represented in the sample, thereby isolating the effect of the factor being studied from the effects of the blocking variable.1