Erik Iverson wrote:
## BEGIN R CODE
## guarantees there is at least one level with exactly three elements,
## which your problem seems to require
t1 <- data.frame(a = rnorm(10), b = c("D", "D", "D",
sample(LETTERS[1:3], 7, replace = TRUE)))
## find which names have exactly three elements
t2 <- subset(t1, b %in% names(which(table(t1$b) == 3)))
## note that the elements of the returned value depend on what was
## originally in your data set's 'b' column
tapply(t2$a, t2$b, mean)
## END R CODE
Hi,
Thank you all for the kind help. Now I've learned much and solved my
problem with your helpful information :)
Cheers,
Leon
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