6.6.1 Correlation

The Pearson correlation coefficient (r) quantifies the strength and direction of a linear relationship between two normally distributed continuous...

The Pearson correlation coefficient (r) quantifies the strength and direction of a linear relationship between two normally distributed continuous variables, ranging from −1 (perfect negative relationship) through 0 (no linear relationship) to +1 (perfect positive relationship); the Spearman rank correlation coefficient provides the non-parametric analogue, based on ranked rather than raw values, and is appropriate for non-normally distributed data or genuinely non-linear but monotonic relationships. It is important to note that correlation quantifies association, not causation, and a statistically significant correlation between two pharmacological variables (for example, plasma drug concentration and a biomarker level) does not by itself establish that one causes the other.