# This R code pertains to the jackknife results of Ch. 10 of E&T.

# First I'll read in the data and compute the y and z values.  
patch <- read.table("http://mason.gmu.edu/~csutton/EandTT101.txt", header=TRUE)
attach(patch)
z <- oldpatch - placebo
y <- newpatch - oldpatch

theta.hat <- mean(y)/mean(z)
theta.hat
# This is the estimate of theta.  ( (10.11) on p. 128 )

# Now to get the jackknife replicates.
ybar.rep <- ( sum(y) - y )/7
zbar.rep <- ( sum(z) - z )/7
th.rep <- ybar.rep/zbar.rep

bias.jk <- 7*( mean( th.rep ) - theta.hat )
bias.jk
# This is the jackknife estimate of bias.  ( (10.32) on p. 134 )

s.e.jk <- sd( th.rep )*7/sqrt(8)
s.e.jk
# This is the jackknife estimate of s.e..  ( (10.35) on p. 137 )

### Here is another way to get the theta hat replicates.
alt.th.rep <- numeric()
for (i in 1:8) alt.th.rep[i] <- mean(y[-i])/mean(z[-i])

# Now I'll check these replicate values by comparing them with the 
# previously obtained replicate values.
compare <- cbind(th.rep, alt.th.rep)
compare