# problem-11.28

problem-11.28 We proceed as follows:

    > 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 *