chapter  8
An introduction to factorial designs and interactions
Pages 19

A factorial design is one in which two or more factors are investigated simultaneously in the same study. Although single-factor designs are useful for illustrating the basic mathematics of Analysis of Variance, they probably form a minority of actual research in psychology. The majority of ANOVA analyses reported in journals have at least two and often three factors. This is because, these days, single-factor designs are often inadequate for shedding light upon the complicated research questions that psychologists are trying to answer. This is usually the case even if a single factor has more than two levels. Designs with two or more factors are therefore very common, especially in cognitive psychology. In theory, they could be analysed by using multiple t-tests but, similarly to single-factor designs with three or more levels, indiscriminate use of pairwise comparisons is inadvisable due to the sheer number of comparisons necessary. Analysis of Variance is generally a more appropriate way of analysing factorial designs.