Objective: To analyse selective factors associated with an unexpectedly low response rate.
Subjects and methods: The baseline questionnaire survey of a large prospective follow-up study on the psychosocial health of the Finnish working-aged randomly chosen population resulted in 21,101 responses (40.0%) in 1998. The non-respondent analysis used demographic and health-related population characteristics from the official statistics and behavioural, physical and mental health-related outcome differences between early and late respondents to predict possible non-response bias. Reasons for non-response, indicated by missing responses of late respondents, and factors affecting the giving of consent were also analysed.
Results: The probability of not responding was greater for men, older age groups, those with less education, divorced and widowed respondents, and respondents on disability pension. The physical health-related differences between the respondents and the general population were small and could be explained by differences in definitions. The late respondents smoked and used more psychopharmaceutical drugs than the early ones, suggesting similar features in non-respondents. The sensitive issues had a small effect on the response rate. The consent to use a medical register-based follow-up was obtained from 94.5% of the early and 90.9% of the late respondents (odds ratio: 1.70; 95% confidence interval: 1.49-1.93). Consent was more likely among respondents reporting current smoking, heavy alcohol use, panic disorder or use of tranquillisers.
Conclusions: The main reasons for non-response may be the predisposing sociodemographic and behavioural factors, the length and sensitive nature of the questionnaire to some extent, and a suspicion of written consent and a connection being made between the individual and the registers mentioned on the consent form.