* GFClocks Example
  Set up a regression model with interaction to
  predict the price of a grandfather clock
  from the age of the clock and number of bids;

gfclocks <- read.table("c:/datasets/gfclocks.txt", header=T)
interact <- gfclocks$age * gfclocks$nbids
model <- lm(price ~ age + nbids + interact, data=gfclocks)
print(summary(model))

pdf("c:/datasets/gfclocks-plots.pdf")

p = predict(model)
r = resid(model)
plot(p, r, main="GFClocks Example -- Interaction Model",
   xlab="Predicted Value", ylab="Residual")
abline(h=0, lty="dashed", col="red")
qqnorm(r, main="GFClocks Example -- Interaction Model")
qqline(r, lty="dashed", col="red")

dev.off( )