an experimental study in which two or more categorical variables are simultaneously manipulated or observed in order to study their joint influence (interaction effect) and separate influences (main effects) on a separate dependent variable. For example, a researcher could use a factorial design to investigate treatment type (e.g., new exercise procedure vs. traditional procedure) and age (＜ 40 vs. ＞ 40). The primary advantages of factorial designs are that they allow for the evaluation of interrelationships and that they are more efficient than conducting multiple studies with one variable at a time. See also fractional factorial design; two-by-two factorial design.