Increasing the number of levels of an independent variable. Eg look at example 10. 1 on page 187. If a curvilinear relationship is predicted, at least three levels must be used. For example, the relationship between fear arousal and performance may such relationship exist in psychology: researchers are often interested in comparing more than two groups. In any given situation a number of variables are operating to affect behaviour eg the experiment in which both the crowding and the windows were effecting the cognitive performance of participants. It is possible to design an experiment with more than one independent variable: factorial designs are designs with more than one independent variable or factor. In a factorial design, all levels of each independent variable are combined with all levels of the other independent variables. In the simplest factorial design, known as a 2 x 2 (two by two) factorial design there are two independent variables each with two levels.