7.1 Introduction

The two-way ANOVA is a method for simultaneously analyzing two factors that affect a response. As in the one-way ANOVA, there is a group effect, such as treatment group or dose level. The two-way ANOVA also includes another identifiable source of variation called a blocking factor, whose variation can be separated from the error variation to give more precise group comparisons. For this reason, the two-way ANOVA layout is sometimes called a 'randomized block design'.

Because clinical studies often use factors such as study center, gender, diagnostic group, or disease severity as a stratification or blocking factor, the two-way ANOVA is one of the most common ANOVA methods used in clinical data analysis.