## ----echo=FALSE----------------------------------------------------------
library(knitr)
opts_chunk$set(size="small",background="white", highlight=FALSE, cache=TRUE, autdep=TRUE, tidy=TRUE, options(show.signif.stars=FALSE))

## ------------------------------------------------------------------------
real.estate <- read.csv("http://www.stat.cmu.edu/~cshalizi/mreg/15/hw/08/real-estate.csv")
# Fit a "kitchen sink" model
  # But don't try to use the ID numbers as a predictor variable!
(realty.lm.all <- lm(Price ~ . -ID, data=real.estate))
step(realty.lm.all, direction="backward", trace=0)

## ------------------------------------------------------------------------
(realty.lm.minimal <- lm(Price ~ 1, data=real.estate))
step(realty.lm.minimal, scope=list(upper = realty.lm.all,
                            lower= realty.lm.minimal), direction="forward",
     trace=0)

## ------------------------------------------------------------------------
step(realty.lm.minimal, scope=list(upper = realty.lm.all,
                            lower= realty.lm.minimal),
     direction="both", trace=0)

## ------------------------------------------------------------------------
(realty.lm.comforts <- lm(Price ~ Pool+Airconditioning, data=real.estate))
step(realty.lm.comforts, scope=list(upper=realty.lm.all,
                             lower=realty.lm.comforts), direction="both",
     trace=0)