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).