Two-group comparisons of patient demographic and
baseline variables were performed with independent
2-sample t tests for continuous variables and χ2 tests of
independence for nominal categorical variables. For continuous
variables with non-normal distributions, and
for ordinal categorical variables, nonparametric Mann-
Whitney U tests were also used. In addition, we calculated
Student t test with unequal variances (Welch’s method).
For the secondary analysis comparing 4 subgroups, 1-way
analysis of variance and Kruskal-Wallis tests were used
in place of the t tests and Mann-Whitney tests. All tests
were 2-tailed. All analyses were done on an intent-to-treat
basis, using SPSS version 18 statistical software (SPSS Inc.,
Chicago, IL).