**Spearman's rank correlation coefficient** or ρ (rho) is a non-parametric measure of correlation – that is, it assesses how well a linear equation describes the relationship between two variables without making any assumptions about the frequency distribution of the variables. To calculate , raw scores are converted to ranks. is a special case of the Pearson product-moment correlation coefficient in which the data are ranks, although a simpler formula is often used in calculation by hand:

*D*= the difference between the ranks of corresponding values of*X*and*Y*, and*N*= the number of pairs of values.