83©FSRH J Fam Plann Reprod Health Care 2010: 36(2) Background

Spearman’s non-parametric correlation coefficient has been used to explore association in the paper by Kulczycki et al.1 in this issue of the Journal. These notes are intended to provide a supplementary explanation of this method. [See Box 1 for a glossary of terms used in this article.] What is non-parametric correlation? A non-parametric (or ordinal) correlation coefficient for two variables quantifies the extent to which there is, in the study sample, a consistent relationship between the values for the two variables. The correlation coefficient indexes the strength of the relationship found, from zero (no discernible relationship) to a maximum value of 1 (denoting a perfect relationship). There are two main methods for obtaining non-parametric or rank correlation coefficients: Spearman rho (ρ) and Kendall tau (τ). Th