Hi R-users,

I have a data.frame like this (modificated from
https://stat.ethz.ch/pipermail/r-help/2007-August/138124.html).

y1 <- rnorm(20) + 6.8
y2 <- rnorm(20) + (1:20*1.7 + 1)
y3 <- rnorm(20) + (1:20*6.7 + 3.7)
y <- c(y1,y2,y3)
x <- rep(1:5,12)
f <- gl(3,20, labels=paste("lev", 1:3, sep=""))
d <- data.frame(x=x,y=y, f=f)

and this is how I can calculate mean of these levels.

tapply(d$y, list(d$x, d$f), mean)

But how can I calculate the mean of d$x 1 and 2 and the grand mean of d$x 1,
2, 3, 4, 5 (within d$f) into a table?

Regards,
Lauri

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