#
# 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.
# 
#-----

library(DAAG)
library(MASS)

lm1 = lm( distance ~ stretch, data=elastic1 )
summary(lm1)

lm2 = lm( distance ~ stretch, data=elastic2 )
summary(lm2) 

print( summary(lm1)$r.squared ) 
print( summary(lm2)$r.squared ) 

# Use robust regression:
#
rlm1 = rlm( distance ~ stretch, data=elastic1 )
rlm2 = rlm( distance ~ stretch, data=elastic2 )

# We follow the books suggestion of plotting the residuals as a function of the fitted values for both the lm and the rlm results:
#
par(mfrow=c(1,2))
plot(lm1,which=1,add.smooth=FALSE)
points( resid(rlm1) ~ fitted(rlm1), col=2, pch=2 ) # use functions : resid and fitted to get the residuals and fitted values for the robust linear models
plot( lm2, which=1, add.smooth=FALSE )
points( resid(rlm2) ~ fitted(rlm2), col=2, pch=2 )
par(mfrow=c(1,1))