Below is an example that illustrates why we should use Kendall's tau instead of Pearson's r. The plot shows group A and B's quality assessments (GDT vs. quality). I'm sure we all agree that the quality assessment of group B is better than group A. However, Pearson's r for group A's assessment is slighter higher than for group B's assessment (0.90 vs 0.89). Actually, a Pearson's r of ~0.9 should