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IntroductionSurveys and registers have provided important insights into the mental health of the community. However, both sources have strengths and limitations. While participation in surveys has been shown to be lower among those with mental disorders, misclassification and limited information on confounders are typical issues for registers.
ObjectivesTo examine if participants of the Central Denmark Region's 2017 How are you? survey were representative of the general population in terms of mental disorder diagnoses.
MethodsBy linking data from the Central Denmark Region's 2017 How are you? survey with the Danish national registers, we compared the frequency of mental disorder diagnoses among (a) participants in the survey (n = 32,417), before and after applying non-response weights, and (b) the entire population who were eligible to participate (n = 1,063,082; 16 years of age or older on 10th January 2017 and registered as living in the Central Denmark Region). Using logistic regression models, we estimated associations between being diagnosed with any mental disorder and nine general medical conditions to assess whether selection into the survey appeared to bias these associations.
ResultsBased on register data, 10.4% (n = 110,492) of the eligible population had received a diagnosis of any mental disorder prior to the date of this survey. Among the unweighted survey sample, 8.2% (n = 2,648) had received a diagnosis; once non-response weights were applied, this corresponded to 9.5%. Representativeness varied by sex, age and type of mental disorder. For example, people with organic disorders or substance use disorders were generally underrepresented among survey participants of all ages; however, representativeness of common disorders such as mood or neurotic disorders was generally good. With respect to the association of any mental disorder and general medical conditions, we found that estimates were similar for survey samples (both weighted and unweighted) compared to the entire eligible population.
ConclusionsPeople with a previous diagnosis of a mental disorder are slightly underrepresented in the survey. However, this selection bias was minimized when non-response weights were applied. Associations between mental disorders and general medical conditions did not appear to be affected by selection bias. With the application of non-response weights, the survey provided a sample representative of the general population in terms of mental disorder diagnoses.

BACKGROUND: Mental disorders account for a substantial proportion of the years lived with disability (YLDs) globally. These estimates have generally been calculated top down based on summary statistics. The aim for this study was to calculate YLDs and a novel related measure, Health Loss Proportion (HeLP), for 18 mental and substance use disorders, based on person-level register data (bottom up). METHODS: A cohort of 6 989 627 Danish residents (5·9% had a diagnosis of a mental or substance use disorder registered in the Danish Psychiatric Central Research Register) was investigated. YLDs (the duration of disease multiplied by a disability weight) were calculated for the disorder of interest and for comorbid mental and substance use disorders. HeLPs were estimated as YLDs associated with an index disorder and comorbid mental and substance use disorders divided by person-years at risk in persons with the index disorder. All analyses were adjusted for mental and substance use comorbidity using a multiplicative model of disability weights. FINDINGS: Major depressive disorder was the most prevalent disorder, although schizophrenia was the leading cause of YLDs in both sexes combined (YLDs 273·3 [95 % CI 232·3-313·6] per 100 000 person-years). People diagnosed with schizophrenia lost the equivalent of 73% (63-83%) of healthy life per year due to mental and substance use disorders, the largest HeLP of all mental and substance use disorders. Comorbidity of mental and substance use disorders accounted for 69-83% of HeLPs in people with either cannabis use disorders, other drug use disorder and ADHD. By contrast, comorbidity explained 11-23% of the HeLPs in people with autism spectrum disorders, conduct disorder, and schizophrenia. INTERPRETATION: Substantial variation in disability was observed across age, sex, and disorders. The new HeLP metric provides novel details of the contribution of comorbidity to the disability associated with mental and substance use disorders. FUNDING: The Danish National Research Foundation, Queensland Government Department of Health, European Union's Horizon 2020, Lundbeck Foundation, Stanley Medical Research Institute. TRANSLATION: For the Danish translation of the abstract see Supplementary Materials section.