Participants and non-participants of different categories in a health survey. A cross-sectional register study.

Participants and non-participants in a health survey of asbestos cement workers were compared, using data from public registers, i.e. variables of socio-medical relevance. The plant closed down in 1976 and 3 years later all living subjects, 985 persons, were offered a health examination. The participation rate was 57%. The non-participants were divided into four categories: I, those who answered the invitation but refused to participate; II, those who did not answer even after three invitations; III, those who answered and accepted but did not participate; and IV, those who participated partly. A very consistent pattern of differences between participants and non-participants was observed, the latter being significantly more often registered for Public Assistance, for heavy drinking habits or criminal offences, and had significantly lower incomes and occupational status. The sickness absence, however, was not significantly shorter among the participants. The non-participating categories II and III differed most from the participants. Category I was more similar to the participants than to the other non-participating categories. This heterogeneity of the non-response/non-participation concept should be considered when health survey data are interpreted.