Determinants of non-participation, and the effects of non-participation on potential cause-effect relationships, in the PART study on mental disorders.

OBJECTIVE: The study had two objectives: (i) to analyse determinants of non-participation in a general population study of mental disorder in Stockholm, and (ii) to determine whether associations between mental disorder and some potential determinants were different among participants and non-participants in the study.
METHODS: The study was based on a questionnaire including potential risk factors for mental disorder and symptom scales. The study group was a random sample of the Stockholm County population aged 20-64 years (19,742 persons). Replies were obtained from 10,441 participants, i. e. the response rate was 53%. The symptom scales were used to identify a group with increased likelihood of mental disorder, screening-positive, and a group with low likelihood of mental disorder, screening-negative. Random samples of the screening-positive and screening-negative respondents were summoned for interview concerning psychiatric symptoms. The database of respondents and non-respondents was linked to several population registers.
RESULTS: The participation was higher in females, among older persons, married persons, among persons with higher income and education and among those born in the Nordic countries. There seemed to be little further risk indicator based selection to interview participation. The associations between in-patient psychiatric care and register variables were strong and similar among participants and non-participants.
CONCLUSIONS: The occurrence of mental disorders is likely to be underestimated in studies with this design and with substantial dropout rates. However, the study participants can most likely be a base for generalising risk indicators for, or social consequences of,mental disorder, to the general population.