Correlation is a statistical method used to analyse the strength of association between variables (Snedecor and Cochran1980). The measure used to describe this relationship is the correlation coefficient (r), a unitless number with values between −1 and 1. Positive values of r describe the tendency of two variables to increase together, while negative values describe the reverse behaviour. In the case where no relationship is found for two variables, the correlation coefficient is zero. Different methods exist to calculate the correlation coefficient. The most common is the Pearson product-moment correlation coefficient. A correlation describes only a statistical association between two variables, not cause and effect.