Statistical Analysis
The sample size was calculated with FEV1 %pred as the
primary outcome. Accepting an alpha risk of 0.05 and a beta risk of 0.2,
we estimated a requirement of at least 90 participants in each group to
recognize an FEV1 %pred difference of ≥ 5% between any
pair of groups as statistically significant. The standard deviation was
assumed to be 10%18.
Descriptive data are expressed as mean, standard deviation (SD), median,
and the 1st and 3rd quartiles (Q) for continuous variables and counts
and percentages for categorical variables. Continuous variables that
followed a normal distribution were compared using a one-way analysis of
variance with post-hoc Bonferroni correction or t- tests. When the
data distribution was not normal, we used the Mann-Whitney U test or
Kruskal-Wallis test with Dunn’s multiple comparison test. Categorical
variables were compared using the chi-squared test or Fisher’s exact
test. To control for potentially confounding variables (allergic
sensitization, GA, parental asthma/atopy, parental smoking, or sex), we
examined differences between groups using multiple linear and logistic
regression models. Adjusted differences and odds ratios with 95%
confidence intervals (95% CIs) were reported. In addition, we performed
a sub-analysis for EP-BPD adolescents and compared participants with
“low” and “high severity” BPD.
p-values < 0.05 or < 0,05/3 for post-hoc tests were
considered statistically significant. Analyses were performed with SPSS
20 (SPSS Inc, Chicago, IL, USA).