Within-Subjects Factorial Design

The within-subjects factorial design is an experimental framework in which each participant is exposed to every combination of two or more manipulated factors, allowing researchers to test the main effect of each factor and their interactions while using each person as their own control. Because the same individuals experience all conditions, between-subject differences are removed from the error term, giving within-subjects factorial designs substantially greater statistical power and efficiency than between-subjects designs for the same number of participants. This makes them a workhorse of experimental social psychology, especially for reaction-time, judgment, and affect studies where many trials per person are feasible. The design's power comes with the need to control order and carryover effects through counterbalancing, and to analyze the data with repeated-measures or mixed-effects models that respect the non-independence of observations from the same person.