2.3. Statistical Analysis
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All the statistical analyses were completed with R software (version 4.2.1). Differences in baseline characteristics between the training and validation groups were analyzed through the Chi-square test. Kaplan–Meier curves along with the log-rank test were used to illustrate and compare the BCSS of patients in the distinct risk groups. The median follow-up time was calculated using the reverse Kaplan–Meier method. The hazard ratios (HRs) and 95% confidence intervals (95% CI) of variables were calculated. HRs are primarily applied in survival analysis to estimate the multiple by which the outcome event risk changes in the exposed group compared to the non-exposed group. The confidence interval refers to the estimated range of a population parameter constructed from sample statistics. The 95% confidence interval indicates that there is a 95% probability that the true value of a population parameter will fall within the interval measured by the sample results. A two-tailed 