| Summary: | Abstract Study samples in medical research are selected according to the objectives of the studies. Researchers seek to collect data as extensively and reliably as possible. In practice, however, data are often missing or may be incorrect. This thesis covers some of the problems concerning missing data and data collection in psychiatric research. Methods for adjusting for missing data and for evaluating the reliability of data are presented. The data originate from the Northern Finland 1966 Birth Cohort (N = 12058). This study explored how participation in an epidemiologic study that includes questionnaires and a clinical examination is affected by mental health (N = 11540), and whether non-participants experience more severe clinical symptoms than participants in a psychiatric field study (N = 145) among subjects with a psychosis. Inverse probability weighting (IPW) was used to adjust for non-participation in comparisons of brain volumes between schizophrenia and control groups. The precision of self-reported medication use was also explored (N = 7625). In an epidemiologic study of all cohort members, subjects with a psychiatric disorder participated less actively than those without one. In the psychiatric field study, non participants were more often patients with schizophrenia than other psychoses. The psychiatric symptoms of non-participants were more severe and they needed more hospital care than participants. The use of IPW led to higher estimates of cerebrospinal fluid volume and lower estimates of grey and white matter volumes in schizophrenia patients, and increased the statistical significance of the differences in brain volume estimates between the schizophrenia and control groups. The precision of self-reported data on psychoactive medication use was substantial. Due to non-participation, the true prevalence of psychiatric disorders is probably higher than the prevalence estimates from field studies that are based on data provided by participants only. In order to reflect the true differences in the ...
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