In Reply In our study,1 we examined the risks of violent victimization and perpetration among patients with psychiatric disorders in Sweden. Clearly, there are strengths and weaknesses of register-based approaches in psychiatric epidemiology. In our study, the strengths outweigh the limitations for the reasons of reliability and consistency of the exposures and outcomes, large sample size for diagnostic subgroups yielding precise estimates, avoidance of certain selection biases, and the ability to use comparison groups, including unaffected siblings. Coid’s first point is that case registers do not have full coverage of psychiatric disorders. However, this is not a unique limitation of case registers because clinical samples often rely on consent and participants remaining in the follow-up (both leading to underrepresentation of patients with severely illness and comorbidities). By using nationwide registers, we had access to reliable, valid, and consistent data on International Classification of Diseases–diagnosed psychiatric disorders that presented to clinical services in the whole country across several decades. This additionally had the advantage of being potentially treatable as they accessed health care services. Some of the limitations with clinical studies are shown in Coid’s work on links between psychiatric morbidity and UK extremism, where the cases to examine the psychiatric disorders were based on 566 pro-British extremists and 47 anti-British extremists.2
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Sariaslan A, Fazel S. Limitations of Case Register Studies for Violence and Psychiatric Disorders—Reply. JAMA Psychiatry. Published online May 06, 2020. doi:10.1001/jamapsychiatry.2020.0835
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