Spearman's rank correlation coefficient (ρ rank ) identifies the ability of the uncertainty estimate to rank the observed errors from low to high. It is calculated by giving the list of uncertainties and the list of absolute errors an integer corresponding to their magnitude i.e. the 10th lowest error gets the value 10. The normal Pearson's correlation coefficient is then calculated for the two ranked lists consisting of integers. The idea in using ranking-based methods such as Spearman's rank correlation coefficient to asses UQ quality is that a lower uncertainty will have a higher probability of low error compared to a higher uncertainty. However, perfect correlation (ρ rank = 1) should not be expected since a high uncertainty can still produce a low error. ρ rank does not take absolute magnitude of the uncertainties into account. For two uncertainties of similar magnitude, there is close to 50% probability that the lower uncertainty will produce a higher error, so that the probability of the uncertainties being ranked "wrong" is high. That probability decreases with increasing difference in the uncertainties. Thus, the distribution of the uncertainties has a big impact on the ability to rank errors according to uncertainties and hence what magnitude of ρ rank we should expect. Perhaps that also explains the differing interpretations of ρ rank existing in the literature. Tynes et al. found ρ rank ranging between 0.2 and 0.65 across tasks for their pairwise difference regression (PADRE) uncertainty estimates and used this as an indication that the uncertainty estimator is "a useful proxy for error across all tasks examined" . Greenman et al. obtained ρ rank = 0.52 for the ensemble variances and took this as suggesting that "one should not necessarily consider the rank order of the prediction uncertainties to be a good approximation of the rank ordering of the prediction errors" . Hirschfeld et al. found ρ rank ranging between -0.17 and 0.34 for the lipophilicity data set when applying a range of UQ methods and concluded that "no method is able to perform particularly well" .