Register-based studies like the current study cannot identify persons with untreated disorders or disorders treated in primary health care only. Most cases with mild to moderate mental disorders, for example, mild or moderate depression and anxiety disorders are thus not registered in the Danish Psychiatric Central Research Register.27 The major strength of the iPSYCH2012 sample approach is the comprehensive clinical assessment of all mental disorders treated in secondary healthcare in a nationwide population. Validation of the Clinician-derived key diagnoses (schizophrenia, single depressive episode, affective disorder, attention-deficit/hyperactivity disorder and autism) has been carried out with good results.83, 84, 85, 86, 87, 88
