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).