Statistical Analysis
Quantitative data following a normal distribution are displayed as mean±Standard deviation and compared using the Student t-test and the ANOVA (Analysis of Variance) for two and three comparisons, respectively. Quantitative data not following a normal distribution are displayed as median (interquartile range) and compared using the Mann-Witney and the Kruskal-Wallis tests for two and three comparisons, respectively. Qualitative variables are displayed as percentages and compared by χ2 or Fisher exact test when appropriate. Univariate Cox regression analysis was used to study the predictive power of the variables studied. Multivariate Cox regression analysis was performed including all variables showing p<0.20 at univariate analysis.
Analyses were performed with R v3.5.1 (R foundation for Statistical Computing, Viena, Austria) and SPSS 19.0 (SPSS Inc., New York).. Statistical significance was set at p<0.05 (two-tailed).