Statistical analysis
The statistical analysis was performed using SPSS Version 20.0 (SPSS Inc., United States).
All of the metric variables were analyzed regarding normal distribution with the Shapiro-Wilk test or the Kolmogorov-Smirnov test. Comparisons of independent normally distributed variables was performed using a t-test.
The homogeneity of variances was tested with the Levene test.
Non-normally distributed samples were compared using the Mann-Whitney-U test.
The categorized data were analyzed with the chi-square test and Fisher’s exact test.
In all of the tests, two-sided significance was tested with the assumption of P < 0.05 as significant.