Spearman rank-order correlation is a nonparametric measure of association based on the ranks of the data values. The formula is
where is the rank of
,
is the rank of
,
is the mean of the
values, and
is the mean of the
values.
PROC CORR computes the Spearman correlation by ranking the data and using the ranks in the Pearson product-moment correlation formula. In case of ties, the averaged ranks are used.