#
# Epage 137
#
# Written by:
# -- 
# John L. Weatherwax                2009-04-21
# 
# email: wax@alum.mit.edu
# 
# Please send comments and especially bug reports to the
# above email address.
# 
#-----

# version$language == "R" for R version$language == NULL for SPlus

if(is.null(version$language) == FALSE){ 
  require(alr3)
}else{
  library(alr3)
}

data(dwaste)
attach(dwaste)

a <- dwaste
a$logO2UP <- log(O2UP)
logO2UP <- a$logO2UP 

# do some data analysis on the scatter plots of this data:
# 
pairs(O2UP~+BOD+TKN+TS+TVS+COD)
plot( TVS, O2UP, xlab="TVS", ylab="O2UP" )

pairs(log(O2UP)~+BOD+TKN+TS+TVS+COD)
plot( TVS, log(O2UP), xlab="TVS", ylab="log(O2UP)" ) 

# the simplest and most complicated model:
# 
m0 <- lm( logO2UP ~ +1 )
m1 <- lm( logO2UP ~ +1+BOD+TKN+TS+TVS+COD ) 

if(is.null(version$language) == FALSE) {require (xtable); xtable(m1)} else m1

# Lets start with forward selection for this problem:
#
ansf1 <- step(m0,scope=list(lower=~+1, upper=~+1+BOD+TKN+TS+TVS+COD), 
                direction="forward", data=a)

ansf2 <- step(m1,scope=list(lower=~+1, upper=~+1+BOD+TKN+TS+TVS+COD), 
                direction="backward", data=a)