Statistical analysis

In this study, the data were analyzed using SPSS software (version 16.0; SPSS Inc., Chicago, IL, USA). The χ² and P-values were calculated to compare whether there were significant differences in frequencies and percentages of the analyzed variables between the breast-cancer case and control groups. Univariate and multivariate Cox regression models, whose effects were equal to those of conditional logistic regression models in the analysis to obtain P-values and odds ratios (ORs) with 95% confidence intervals (95% CIs) of the variables. The statistical significance was set at a two-sided P-value of 0.05.