Statistical analyses
Statistical analyses were performed with the aid of SPSS ver. 21 (SPSS Inc., IBM, Chicago, IL). Data are expressed as means (with standard deviations [SDs]) or percentages (%). The chi-squared test was used to compare categorical data. The Wilcoxon and Mann–Whitney U-tests were employed to compare non-parametric variables and the independent and paired samples t-tests were used to compare parametric variables. A 