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)

*Equation*