ABSTRACT
"Two-factor ANOVA with interaction" does not imply that interaction actually occurs; it merely means that interaction is considered in the calculation. The name "two-factor ANOVA without interaction" does not imply that there is no interaction; it merely means that interaction is not examined in the calculation. This chapter considers the two-factor ANOVA with interaction, and examines exactly what we mean by "interaction". Interaction takes place between factors. Two factors interact if the trend between treatments in one factor varies depending on which treatment is present in the other factor. Just because there is no between-treatments effect for a given factor, it does not mean that the treatment means are always the same for each condition of the other factor; interaction indicates whether they are not. The pooling of insignificant interactions with the error is only a tradition and is not customary with all researchers.
