2.4. Statistical analysis
The categorical variables are expressed as absolute values and percentages and were compared using a chi-squared test. The continuous variables are expressed as mean ± SD and were compared using a t-test. Univariate and multivariate Cox proportional hazards analysis were applied to study the determinants of post-operative AA events among the parameters of clinical, echocardiographic and surgical data. Multivariate analyses were performed using stepwise selection with models using significant factors from univariate analyses. Sensitivities, specificities, positive predictive values, negative predictive values and accuracies for predicting AA events were calculated using standard methods. Receiver operating characteristic (ROC) curves and the areas under the ROC curves were assessed using standard methods. The optimal cut-off value was defined as that with highest sum of sensitivity and specificity. Survival curves were produced according to the Kaplan-Meier method, and differences between groups were compared with the log-rank test. A P-value < 0.05 was considered statistically significant. All statistical analyses were performed with EZR (Saitama Medical Center, Jichi Medical University, Saitama, Japan), which is a graphical user interface for R (The R Foundation for Statistical Computing, Vienna, Austria). More precisely, it is a modified version of R commander designed to add statistical functions frequently used in biostatistics.