The continuous variables are given as mean values ± standard deviation (SD), and were analysed using a two-sided, non-parametric Wilcoxon rank-sum test or, when the data were normally distributed (on the basis of the Shapiro-Wilks statistic), a two-sided Student's t-test; the categorical variables are given as absolute numbers and percentages, and were compared between groups using contingency table analysis with the χ 2 test or Fisher's exact test as appropriate.