#
# 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)

lm_nihills = lm( log(time) ~ log(dist) + log(climb), data=nihills )
yhat_hills2k_from_nihills = predict( lm_nihills, newdata=hills2000 )

lm_hills2000 = lm( log(time) ~ log(dist) + log(climb), data=hills2000 )
yhat_hills2k_from_hills2k = predict( lm_hills2000, newdata=hills2000 )

plot( yhat_hills2k_from_hills2k, yhat_hills2k_from_nihills, xlab="hills2k predicted from hills2k model", ylab="hill2k predicted from nihills model" )

# True log(time) minus predicted log(times) from model of nihill data: 
var_using_nihills = var( log(hills2000$time) - yhat_hills2k_from_nihills )

# True log(time) minus predicted log(times) from model of hills2000 (should be smaller): 
var_using_hills2000 = var( log(hills2000$time) - yhat_hills2k_from_hills2k )

print( sprintf("var using nihills= %10.6f; using hills2k= %10.6f", var_using_nihills, var_using_hills2000) )