The clinical cohort consists of individuals who have been referred to a child psychiatric outpatient clinic in the Northern Netherlands any time before the age of 11. Data collection in this cohort started a few years after the population cohort, in 2004, with the inclusion of 543 children (response rate 43%).1 Boys predominated in the clinical cohort because they were overrepresented in the most prevalent diagnostic groups in the outpatient clinic (i.e. attention deficit / hyperactivity disorder, disruptive behaviour and autism-spectrum disorders). Comparable to the population cohort, follow-up data collection waves occurred at intervals of 2–3 years. At present, four waves have been completed in the clinical cohort, with satisfactory retention rates (Table 1).
