options(digits=4)
rats=read.csv("S:/webpages/~renaes/Data/weight gain.csv",header=T)
attach(rats)

boxplot(gain~type+level,main="Boxplot of Weight Gain in Rats by
        Food Type and Level") 

rat.fit=lm(gain~type*level)
anova(rat.fit)

interaction.plot(level,type,gain, type="b",main="Interaction Plot
                 of \nWeight Gain in Rats by Food Type and Level") 

# checking assumptions:

res=rstudent(rat.fit)
pred=fitted.values(rat.fit)
hist(res,main='Histogram of Residuals')
plot(pred,res,main='Predicted vs. Residuals')
abline(0,0); abline(-2,0,col='red'); abline(2,0,col='red')
qqnorm(res,xlim=c(-3,3),ylim=c(-3,3)); qqline(res)