ABSTRACT
This chapter extends the mixed-model methodology for one-way within-subjects designs to more complicated factorial designs. In any two-way design, the initial questions typically of most interest are the significance of the two main effects and the interaction. In other words, the effects to be tested are the same whether the factors are within or between subjects. The meaning to be attached to significant effects in the two-way within-subjects design is the same as in the two-way between-subjects design. As in between-subjects factorial designs, the most frequent method of interpreting a statistically significant interaction is to perform tests of simple effects. As in other factorial designs, the most typical method of interpreting a statistically significant interaction in a split-plot design is to perform tests of simple effects.
