Recall that a between-subjects experimental design requires a separate, independent group of individuals for each treatment condition. Because the goal is to infer any differences in the dependent variable between conditions as due to the independent variable, it is important to control for the presence of any confounding variables that might account for such differences across conditions. Individuals are assigned to conditions using a procedure that attempts to create equivalent groups, that is, random assignment. Through this procedure each subject has “an equal and independent chance of being assigned to every condition” (McBurney & White, 2004, p. 196). Between-subjects designs often require more subjects to complete the study than within-subjects designs; however, between-subjects designs are often necessary when order or sequence effects make within-subjects designs impossible.