In factorial designs, the study of the interaction allows the researcher to determine whether the effect of one factor depends on the level of the other factors. In order to do so, the usual method is to consider the differences among the mean values resulting from each single-factor experiment. The present article proposes that the ratio among the mean values should also be considered, when the level of measurement of the dependant variable allows it. This makes possible the study of a kind of interaction that has not been considered so far, which is described under the name of ratio interaction. This paper discusses the usefulness of considering the ratio interaction.