Statistical Analysis
BPD
Analyses included all randomized and surviving children from 5 Australian hospitals involved in the 5-year follow-up phase of the trial (656 children). Statistical calculations were performed using Stata statistical software version 17 (StataCorp), with a 2-tailed Preliminary analyses were undertaken to describe the total effect of DHA supplementation on IQ, as reported previously,For the mediation analysis, we used a simulation-based A complicating factor for analysis was the extent of missing data for IQ. As the probability of missing data depended on observed characteristics in the data set, multiple imputation implemented under a missing-at-random assumption was used to address missing data. Imputation was performed separately by randomization group using chained equations,The robustness of findings to assumptions about the missing data, underlying causal model, and approach to defining BPD was investigated in sensitivity analyses. For missing data, we considered missing not at random mechanisms via pattern mixture models, with IQ scores assumed to be 3 points lower or 3 points higher in children with missing data compared with children who had observed data. This difference was viewed as a very large departure from the 0-point difference expected under a missing-at-random assumption. We did not undertake sensitivity analyses for the imputation of BPD given that this measure had a low level of missing data. For the assumed causal model, we considered a sensitivity analysis with small for gestational age included as an additional confounder of the BPD-IQ association. This confounder was defined as a birthweight less than the 10th percentile given sex and gestational age.