[R] paired t-test. Need to rearrange data?
Henrik Parn
henrik.parn at bio.ntnu.no
Sun Aug 6 16:13:29 CEST 2006
Dear all,
I have received some data arranged like this:
# original data
id <- rep(letters[1:6], each=2)
time <- as.factor(rep(1:2, 6))
y <- as.vector(replicate(6, c(rnorm(n=1, mean=1), rnorm(n=1, mean=2))))
test.data <- data.frame(id, time, y)
test.data
I would like to perform a paired t-test of the y-values at time=1
against those at time=2, with samples paired by their id. Is it
necessary to arrange the data in a format like this:
# rearranged data
id <- letters[1:6]
y1 <- replicate(6, rnorm(n=1, mean=1)) # y-value at time = 1
y2 <- replicate(6, rnorm(n=1, mean=2)) # y-value at time = 2
test.data2 <- data.frame(id, y1, y2)
test.data2
...in order to perform a paired t-test?
t.test(y1, y2, paired=T)
If yes, which is the most convenient way to rearrange the data?
Or is it possible to apply the paired t-test function to the original
data set?
And a side question: In my examples, I suppose can I use set.seed to
reproduce the 'rnorm-values' created in the 'original data' also in my
the 'rearranged data'. Can someone give me a hint of how to apply the
same 'seed' to all the rnorms?
Thanks a lot in advance!
Henrik
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