Statistical methods
All statistical analyses were performed with SPSS statistical software
version 21.0 (IBM Corp, Somers, NY, USA). Continuous values are
expressed as mean ± standard deviation or standard error of mean (SEM)
as indicated and were compared with Student’s t-test after confirmation
of normal distribution. Otherwise, the Mann-Whitney-U-test was used.
Categorical variables are displayed as frequency and percentages and
were compared using the Chi-square-test or Fisher’s exact test in small
sample sizes or when one or more of the cells had an expected frequency
of five or less. Multi-group comparisons were performed using ANOVA test
with Bonferroni post-hoc correction. For intra-group comparisons, a
paired t-test was used. A p-value of less than 0.05 was considered to
indicate statistical significance.