Given the non-normality of the h index (skew = 1.88 and kurtosis = 8.24), non-parametric statistical tests were employed. Depending on the number of samples and the nature of their relationships, these tests included the Kruskal–Wallis test for comparison of multiple independent samples and the Wilcoxon signed ranks test for comparison of related samples ( Fahoome, 2002). Relationships were quantified with the Spearman rank order correlation coefficient and complex relationships were analyzed with quantile regression.