Social selection in cohort studies and later representation of childhood psychiatric diagnoses: The Danish National Birth Cohort

Aim: This study aimed to estimate the relative representation of childhood psychiatric diagnoses and use of psychotropic medication in the Danish National Birth Cohort (DNBC) compared to the general population. Methods: The general population was identified as all childbirths in Denmark during 1998–2002 (N=344,160). Linking the DNBC (N=91,442) and the general population to the Danish national health registries, all children were followed until they received an ICD-10 psychiatric diagnosis, had a prescription of psychotropic medication or to the end of follow-up in 2013. The prevalence ratios (PRs) with corresponding 95% confidence intervals (CI) were estimated for each psychiatric diagnosis and by sex. Age at first diagnosis presented as means were compared using the one-sample t-test. Results: In the DNBC, the selected childhood psychiatric diagnoses were underrepresented by 3% (PR=0.97, 95% CI 0.94–0.99), ranging from a 20% underrepresentation for schizophrenia (PR=0.80, 95% CI 0.59–1.09) to a 6% over-r...