# 
# 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(alr3)
data(wblake)
attach(wblake)
plot(Age,Length)
abline(lm(Length~Age))

# plot the means on top of the data: 
uAge <- sort(unique(Age))
lines(uAge,tapply(Length,Age,mean))

# compute the sd as a function of age ... looks very much like a null plot
lsd <- tapply(Length,Age,sd)
plot(1:8,lstd)