APA Dictionary of Psychology

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.