Statistical analysis.
Descriptive statistics are presented as mean and standard deviation . Group comparisons were performed using t-test, Mann-Whitney U-test, and chi2-test, as appropriate. In order to identify factors independently associated with BPD severity levels, we fit the data to partial proportional odds models for ordinal dependent variables (ordinal logistic regression). Candidate confounders were selected based on a review of the literature and biological plausibility. Regression results are reported as odds ratios and their respective 95% confidence intervals (CI). All statistical tests were two-tailed, and the significance level used was p < 0.05. The data were analyzed with the Statistical Package Stata 12.0 (Stata Corporation, College Station, TX, USA).