Use Pearson's product-moment correlation coefficient when two quantitative variables have an approximately linear relationship. Use Spearman's rank correlation coefficient when the data are ordinal or when the relationship is monotonic but not necessarily linear.
Pearson's coefficient, , is calculated from the original numerical values. It measures the strength and direction of linear association, so the scatter diagram should resemble a straight-line pattern.
Spearman's coefficient, , is calculated from the ranks of the values. A relationship is monotonic if one variable generally increases as the other increases, or generally decreases as the other increases, even when the pattern is curved.
