Statistical analysis
Categorical variables were expressed as frequencies and percentages (%), and continuous variables were expressed as mean ± standard deviation (SD). The Shapiro–Wilk test was used to assess the normality assumption for the continuous variables. The Student’s t-test or Mann–Whitney U test was used between groups, whether the statistical hypotheses were fulfilled or not. The Chi-squared test was used to compare the distribution of categorical variables in groups. Simple logistic regression was used for bivariate analysis to identify individual associations with stone-free status (Yes/No), hospitalization time (1 day versus more than 1 day) and postoperative complications (Yes/No). A p value of < .25 was accepted as significant in the selection of the factors whose efficacy would be assessed with multiple logistic regression analysis, which was performed stepwise. Significant results are presented as odds ratios (ORs) and 95% confidence interval (CI). Multiple regression was run to predict operation time. Non-significant covariates were removed from the model. There was linearity as assessed by a plot of studentized residuals against the predicted values. The area under the receiver operating curve (AUC) was calculated. All statistical analyses were performed using SPSS 19.0 software (IBM Corp., Armonk, NY, USA). All statistical tests had a statistical significance level of p value < .05 (two-tailed).