title: ADHD, comorbid disorders and psychosocial functioning: How representative is a child cohort study? Findings from a national patient registry.
abstract: Cohort studies often report findings on children with Attention Deficit Hyperactivity Disorder (ADHD) but may be biased by self-selection. The representativeness of cohort studies needs to be investigated to determine whether their findings can be generalised to the general child population. The aim of the present study was to examine the representativeness of child ADHD in the Norwegian Mother and Child Cohort Study (MoBa).
  
  
  
  The study population was children born between January 1, 2000 and December 31, 2008 registered with hyperkinetic disorders (hereafter ADHD) in the Norwegian Patient Registry during the years 2008-2013, and two groups of children with ADHD were identified in: 1. MoBa and 2. The general child population. We used the multiaxial International Classification of Diseases (ICD-10) and compared the proportions of comorbid disorders (axes I-III), abnormal psychosocial situations (axis V) and child global functioning (axis VI) between these two groups. We also compared the relative differences in the multiaxial classifications for boys and girls and for children with/without axis I comorbidity, respectively in these two groups of children with ADHD.
  
  
  
  A total of 11 119 children were registered with ADHD, with significantly fewer in MoBa (1.45%) than the general child population (2.11%), p < 0.0001. The proportions of comorbid axis I, II, and III disorders were low, with no significant group differences. Compared with the general child population with ADHD, children with ADHD in MoBa were registered with fewer abnormal psychosocial situations (axis V: t = 7.63, p < .0001; d = -.18) and better child global functioning (axis VI: t = 7.93, p < 0.0001; d = .17). When analysing relative differences in the two groups, essentially the same patterns were found for boys and girls and for children with/without axis I comorbidity.
  
  
  
  Self-selection was found to affect the proportions of ADHD, psychosocial adversity and child global functioning in the cohort. However, the differences from the general population were small. This indicates that studies on ADHD and multiaxial classifications in MoBa, as well as other cohort studies with similar self-selection biases, may have reasonable generalisability to the general child population.
authors.affiliation: Division of Mental Health and Addiction, Oslo University Hospital, Pb. 4959 Nydalen, 0424, Oslo, Norway. beate.orbeck@ous-hf.no.
authors.forename: Beate
authors.initials: B
authors.lastname: Oerbeck
authors.affiliation: Division of Mental Health and Addiction, Oslo University Hospital, Pb. 4959 Nydalen, 0424, Oslo, Norway.
authors.forename: Kristin Romvig
authors.identifier: 0000-0002-8150-9030
authors.initials: KR
authors.lastname: Overgaard
authors.affiliation: The Norwegian Patient Register, Trondheim, Norway.
authors.forename: Stian Thoresen
authors.initials: ST
authors.lastname: Aspenes
authors.affiliation: Oslo University Hospital, Oslo Centre of Biostatistics and Epidemiology, Oslo, Norway.
authors.forename: Are Hugo
authors.initials: AH
authors.lastname: Pripp
authors.affiliation: Division of Mental Health and Addiction, Oslo University Hospital, Pb. 4959 Nydalen, 0424, Oslo, Norway.
authors.forename: Marianne
authors.initials: M
authors.lastname: Mordre
authors.affiliation: Division of Mental Health, Norwegian Institute of Public Health, Oslo, Norway.
authors.forename: Heidi
authors.initials: H
authors.lastname: Aase
authors.affiliation: Division of Mental Health, Norwegian Institute of Public Health, Oslo, Norway.|Institute of Clinical Medicine, University of Oslo, Oslo, Norway.
authors.forename: Ted
authors.initials: T
authors.lastname: Reichborn-Kjennerud
authors.affiliation: Division of Mental Health and Addiction, Oslo University Hospital, Pb. 4959 Nydalen, 0424, Oslo, Norway.
authors.forename: Pal
authors.initials: P
authors.lastname: Zeiner
mesh_terms: D001289:Attention Deficit Disorder with Hyperactivity / Q000175:diagnosis / Q000453:epidemiology* / Q000523:psychology*; D002648:Child; D015331:Cohort Studies; D015897:Comorbidity; D005260:Female; D006801:Humans; D038801:International Classification of Diseases; D007398:Interpersonal Relations; D008297:Male; D009035:Mothers / Q000523:psychology*; D065886:Neurodevelopmental Disorders / Q000175:diagnosis / Q000453:epidemiology / Q000523:psychology; D009664:Norway / Q000453:epidemiology; D011446:Prospective Studies; D012042:Registries
keywords: ADHD; Child psychiatry; ICD-10; MoBa; Multiaxial international classification of Diseases; National patient registry