Using R for Introductory Statistics : John Verzani

Using R for introductory statistics 324It matches up, although we didn’t fuss with the order.Now to see if an interaction term is warranted:> interaction.plot(Seat, Popcorn, x)‡ See ?xtabs for a similar example.Figure 11.8 Interaction plotindicating presence of aninteraction, as lines are not parallelFigure 11.8 seems to show an interaction, as the slopes are not parallel. We can do aformal test with anova().> res.int=1m(x ~ Seat * Popcorn)> res.add=lm(x ~ Seat + Popcorn)> anova(res.int,res.add)Analysis of Variance TableModel 1: x ~ Seat * PopcornModel 2: x ~ Seat + PopcornRes.Df RSS Df Sum of Sq F Pr(>F)1 12 2772 13 638 −1 −361 15.6 0.0019 **The small p-value casts doubt on the null hypothesis model of no interaction. Thesummary() function gives more detailed information about the interaction model.> summary(res.int)Call:1m(formula = x ~ Seat * Popcorn)--Coefficients:Estimate Std. Error t value Pr(>|t|)