The impact of study design on schizophrenia incidence estimates: A systematic review of Northern European studies 2008–2019

The best estimates of the incidence of schizophrenia range more than 25-fold from 3 to 80 per 100,000 person-years. To what extent do differences in study design explain this wide variation? We selected all studies published between 2008-2019 reporting the incidence of schizophrenia in general populations of Northern Europe. We identified 17 estimates covering 85 million person-years and more than 15,000 individual cases. The estimates ranged from 4-72 per 100,000 person-years (median 30; interquartile range 13-41). We classified the estimates in terms of three study design factors (coverage of services, time frame, and diagnostic quality) and two population factors (urbanicity and age). A meta-regression model of the three design factors, using the two population factors as covariates, explained 91% of between-study variation. Studies performed in general psychiatric services reported similar estimates [incidence rate ratio 1.12 (95% confidence interval 0.88 to 1.43)] to those performed in specialized services. But studies applying a cumulative time frame to diagnosis reported fourfold higher estimates [4.04 (3.14 to 5.2)] than those applying a first-contact time frame. And studies based on clinical diagnoses reported lower estimates [0.55 (0.43 to 0.72)] than those based on standardized research diagnoses. The three study design factors by themselves explained 67% of between-study variation. When comparing incidence rates from different populations, distorsions arising from differences in study design can eclipse differences caused by schizophrenia risk factors, such as gender, age or migrant status.