spearman correlation by group in R

by

How about this for a base R solution:

df <- data.frame(group = rep(c("G1", "G2"), each = 10),
                 var1 = rnorm(20),
                 var2 = rnorm(20))

r <- by(df, df$group, FUN = function(X) cor(X$var1, X$var2, method = "spearman"))
# df$group: G1
# [1] 0.4060606
# ------------------------------------------------------------ 
# df$group: G2
# [1] 0.1272727

And then, if you want the results in the form of a data.frame:

data.frame(group = dimnames(r)[[1]], corr = as.vector(r))
#   group      corr
# 1    G1 0.4060606
# 2    G2 0.1272727

EDIT: If you prefer a plyr-based solution, here is one:

library(plyr)
ddply(df, .(group), summarise, "corr" = cor(var1, var2, method = "spearman"))

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