3.6 Statistical analysis
Categorical values are expressed as absolute values and relative frequencies. Continuous variables are expressed as mean ± SD. Tests for significance were conducted using the unpaired t-test or nonparametric test (Mann-Whitney U-test) for continuous variables and the chi-squared test or Fisher’s exact test for categorical variables. The cutoff value of the voltage in each region was examined using the Receiver Operating Characteristic (ROC) curve. The score below the cutoff value was set as 1 point each, and the relationship with the recurrence of atrial fibrillation was examined by a chi-square test. Clinical factors associated with AF recurrence were determined using univariate and multivariate Cox proportional hazard models and logistic regression analyses. All statistical analyses were performed using commercial software (SPSS version 25 ®, SPSS, Chicago, IL, USA), and statistical significance was defined as p <0 .05.