title: Non-participation may bias the results of a psychiatric survey: an analysis from the survey including magnetic resonance imaging within the Northern Finland 1966 Birth Cohort.
abstract: OBJECTIVE
  A major reason for limited validity of research is non-participation. Subjects with severe mental illness tend to cumulate in the group of non-participants, causing selection bias. We tested the hypothesis that severe psychosis is linked to non-participation in a field survey including magnetic resonance imaging (MRI) of the brain, psychiatric interviews and cognitive testing among subjects with psychosis. Furthermore, we wanted to explore other associative factors expected to affect non-participation.
  
  
  METHODS
  Members of the Northern Finland 1966 Birth Cohort with a lifetime diagnosis of psychosis (N = 145) were invited to participate in the survey conducted in 1999-2001. Non-participation was determined by refusal or loss of contact. Data were gathered in earlier phases of the follow-up study or using register data.
  
  
  RESULTS
  Ninety-one (63%) subjects attended the study. Compared to participants, non-participants were more often patients with schizophrenia and had more psychiatric hospitalisations, they had more positive psychosis symptoms during their illness course and they were more often on disability pension. Married subjects participated more often than those who were not married.
  
  
  CONCLUSIONS
  This study suggests that among subjects with psychosis, particularly those subjects who have the most severe course of illness are less willing to participate. This may lead to biased estimates when studying subjects with severe mental disorders.
authors.affiliation: Department of Psychiatry, University of Oulu, Oulu, Finland. marianne.haapea@oulu.fi
authors.forename: Marianne
authors.initials: M
authors.lastname: Haapea
authors.forename: Jouko
authors.initials: J
authors.lastname: Miettunen
authors.forename: Juha
authors.initials: J
authors.lastname: Veijola
authors.forename: Erika
authors.initials: E
authors.lastname: Lauronen
authors.forename: Päivikki
authors.initials: P
authors.lastname: Tanskanen
authors.forename: Matti
authors.initials: M
authors.lastname: Isohanni
mesh_terms: D000328:Adult; D017668:Age of Onset; D015982:Bias; D002391:Catchment Area, Health; D015331:Cohort Studies; D003256:Community Participation / Q000706:statistics & numerical data*; D005260:Female; D005387:Finland / Q000453:epidemiology; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000706:statistics & numerical data*; D008297:Male; D011570:Psychiatry / Q000706:statistics & numerical data; D012042:Registries; D012559:Schizophrenia / Q000453:epidemiology* / Q000534:rehabilitation; D012720:Severity of Illness Index; D011795:Surveys and Questionnaires