My question was: I'm trying to calculate the correlation between "ftd" and "ici" variables but I can't figure out how to use cortab function and I couldn't find any example in web about that.
To those who want to learn how to use cortab function, thanks to Giovanni Millo (one of the authors of plm package), I found examples. I've published the database of it and just in case I'm also putting the codes here.
Here is a toy example of the cortab function in package plm:
library(plm)
data("Grunfeld", package = "plm")
pGrunfeld <- pdata.frame(Grunfeld)
grp <- c(rep(1, 100), rep(2, 50), rep(3, 50)) # make 3 groups
cortab(pGrunfeld$value, grouping = grp, groupnames = c("A", "B", "C"))
# A B C
# A 0.5562310 NA NA
# B 0.5431652 0.5406152 NA
# C 0.3799585 0.2999645 -0.1665637
The output matrix contains the average cross–sectional correlation coefficients within a group (diagonal) and between groups (off-diagonal).