Table13 <- read.csv("https://sullystats.github.io/Stats7e/Data/Ch13/Table13.csv")
head(Table13)
##        Age    Drug HDL
## 1 18 to 39 Placebo   4
## 2 18 to 39 Placebo   3
## 3 18 to 39 Placebo  -1
## 4 18 to 39    5 mg   9
## 5 18 to 39    5 mg   5
## 6 18 to 39    5 mg   6

Now, run a two-way ANOVA treating HDL as the response variable and Age & Drug as the treatments. Note that we include the interaction term Age:Drug in the analysis.

summary(aov(HDL ~ Age + Drug + Age:Drug,data=Table13))
##             Df Sum Sq Mean Sq F value   Pr(>F)    
## Age          1  14.22   14.22   3.507   0.0857 .  
## Drug         2 208.33  104.17  25.685 4.61e-05 ***
## Age:Drug     2   8.78    4.39   1.082   0.3697    
## Residuals   12  48.67    4.06                     
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

The P-value for the interaction term is 0.3697. The P-value for Age is 0.0857 and the P-value for Drug is 4.61e-05 = 4.61 x \(10^{-5}\) = 0.0000461.