How are treatments assigned in a completely randomized design?

In a completely randomized design, treatments are assigned to experimental units through a process of random assignment. This method ensures that each unit has an equal opportunity to receive any of the available treatments, which helps to eliminate selection bias and manage confounding factors that might otherwise influence the response variable.12

For example, if a study involves testing three different exercise programs on patients, each patient is randomly assigned to one of the three programs. By relying on chance to distribute the treatments, researchers aim to ensure that the groups are comparable, thereby strengthening the validity of the conclusions drawn from the experiment. While this design is straightforward, it may yield less precise results if there are external factors—such as inherent differences between subjects—that are not accounted for by the experimenter. In such cases, researchers might instead use a randomized block design to control for those specific variables.12