> with(ToothGrowth, interaction.plot(dose, supp, len)) > res.full = lm(len ~ supp + dose, ToothGrowth) > res.add = lm(len ~ supp * dose, ToothGrowth) > anova(res.full, res.add) Analysis of Variance Table Model 1: len ~ supp + dose Model 2: len ~ supp * dose Res.Df RSS Df Sum of Sq F Pr(>F) 1 57 1023 2 56 934 1 89 5.33 0.025 *