Statistical analysis
Continuous variables were analysed using the independent t-test or Mann–Whitney U-test depending on the normality of the distribution. Normality was examined by the use of the Shapiro–Wilk test. Categorical variables were analysed by Pearson’s chi-squared test or Fisher’s exact test, as appropriate. All statistical tests used a two-tailed α of 0.05. A P value < 0.05 was considered statistically significant. Analyses were performed using STATA 13.0 (StataCorp. Stata Statistical Software: Release 13. CollegeStation, TX, USA).