Statistical analyses were performed using Statistical Analysis System (SAS Institute Inc., version 9.1, Enterprise Guide 3.0, Cary, NC, USA) and R (version 2.7.2) by a statistician in our hospital. The Fisher's exact test was used for frequency data. The variables presenting normal distribution were compared by t test or analysis of variance. For the variables which did not show normal distribution, the Kruskal-Wallis test or Wilcoxon test were used. A P-value <0.05 was considered statistically significant.