Random assignment is a technique for assigning participants to experimental conditions and a prerequisite of true experimental designs. It requires the use of randomization methods to place participants of a particular study into experimental conditions (e.g., treatment vs. control). It ensures that each participant has an equal chance of being placed into either of the experimental groups.

Systemic differences at the outset of an experiment can hurt internal validity—the degree to which effects of the experiment can be attributed solely to the experimental treatment. The random assignment of study members into groups is a requirement for alleviating any initial systemic differences between experimental groups. However, random assignment alone is not a guarantee that there won’t be any initial differences between groups, but rather that any initial ...