eAppendix 1. Multiple imputation of missing covariate data
Data was missing for three covariates: Maternal ethnicity was not registered in the Danish Medical Birth Registry in 1995-1996. From 1997-2003, only 99 observations were
missing. We also lacked information on maternal education for all children born in 2003;
otherwise maternal education was missing for 2.5 % of births. Maternal smoking was missing for 4.4 %.
Information on maternal mental disease and birth hospital was included in the imputation model as predictors of the missing observations. Since registration in the Medical Birth Registry is performed immediately after birth, adverse perinatal outcomes (low apgar score, low birthweight, preterm birth, induced or instrumental labor, and cesarean section) were included as predictors of missing observations from this register.
Additionally, all variables in the analyses were included in the imputation model as well as the Nelson-Aalen estimate as a proxy of the cumulative hazard function1. By the use of chained equations in Stata 14 (StataCorp, TX, USA), 100 new datasets were imputed.
The distribution of maternal smoking, ethnicity, and education in the imputed data was as expected based on the predictive variables.
1.White IR, Royston P, Wood AM. Multiple imputation using chained equations: Issues and guidance for practice. Stat Med. 2011;30(4):377-399.
