2.6 Statistical analysis
Continuous variables were expressed as mean ± standard deviation. Discrete variables were presented as percentages. Univariate comparisons among the variables were made using the chi-squared (χ2) test for categorical variables and the Student’s t test for continuous variables. A one-way ANOVA followed by Bonferroni’s post hoc test was performed to evaluate the statistical differences of variables among the groups for continuous variables. Pearson correlation analysis was performed for multiple continuous variables. All tests were two-tailed. P < 0.05 was considered statistically significant. All statistical analyses were performed by SPSS (SPSS Inc., version 21.0, Chicago, IL, USA).