All data were entered into a Microsoft Access 2000 database and, after plausibility checks, transferred into SPSS 15.0 for statistical analysis. Data were analysed descriptively (percentages or median with inter-quartile range, IQR; reported percentages refer to the number of available data per variable). Comparisons among groups were assessed for significance (p < 0.05, two-sided) using Pearson's Chi 2 -test or, where appropriate, Fisher's Exact test for categorical data. Continuous data were assessed using the Mann-Whitney U-test. Due to the small number of cases per subgroup, the evidence of the statistical comparisons was limited.