Spearman's Rho is a non-parametric test used to measure the strength of association
between two variables, where the value r = 1 means a perfect positive correlation
and the value r = -1 means a perfect negataive correlation. So, for example,
you could use this test to find out whether people's height and shoe size are correlated
(they will be - the taller people are, the bigger their feet are likely to be).

Requirements

Scale of measurement must be ordinal (or interval, ratio)

Data must be in the form of matched pairs

The association must be monotonic (i.e., variables increase in value together, or
one increases while the other decreases)