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 *