### R code for Section 7.1

## Example 7.1-1: Popcorn

## The yield (in cups of popped corn from 1/4 cup of corn) is the variable of interest
## Wish to test the effects of two factors: 
## Factor A is the type of popcorn maker, oil-based or air-based
## Factor B is popcorn brand, gourmet-type and a national brand

data=read.table("http://homepages.math.uic.edu/~wangjing/stat481/table715.dat", head=T)

yield = data$Yield
popper = factor(data$Popper)
brand = factor(data$Brand)

# linear regression
fit1 = lm(yield ~ popper + brand + popper*brand)
summary(fit1)


par(mfrow=c(2,2))
# residual plot
plot(fit1, 1)    # residual vs fitted value
plot(fit1, 2)    #  QQ-plot 
plot(fit1, 3)    # sqrt(|standardized residual|) vs FItted value
plot(fit1, 4)    # cook's distance

# ANOVA table
anova(fit1)

# check row means
tapply(yield, popper, mean)
# check column means
tapply(yield, brand, mean)


# interaction plot
par(mfrow=c(1,1))
interaction.plot(brand, popper, yield)

# Strip chart (dotplot)
stripchart(yield ~ brand, method = "stack", pch=1)
stripchart(yield ~ popper, method = "stack", pch=1)