# 
# 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.
# 
#-----

save_plots = F

#
# EPage 140
#

DF = read.csv( "../../Data/used_car_prices.csv", header=TRUE )

# Show year is important in determining price but mileage is not:
#
m = lm( Price ~ Year + Mileage, data=DF )
summary(m)

# Refit our model using only Year:
#
m = lm( Price ~ Year, data=DF )
summary(m)

# Perform the regression for each model class:
#
for( l in levels(DF$Class) ){ 
    print( l )
    print( summary ( lm( Price ~ Year + Mileage, data=DF[ DF$Class==l, ] ) ) )
}

# Determine if color is important:
#
DF$isBlack = 0
DF$isBlack[ DF$Color == 'black' ] = 1

m2 = lm( Price ~ Year + isBlack, data=DF )
summary(m2)

if( save_plots ){ postscript("../../WriteUp/Graphics/Chapter6/ucp_residual_plot.eps", onefile=FALSE, horizontal=FALSE) }
plot(m, which=1)
if( save_plots ){ dev.off() }