Late response and item nonresponse in the Finbalt Health Monitor survey.

BACKGROUND The Finbalt Health Monitor is a collaborative system for monitoring the health-related behaviour, practices and lifestyles in Estonia, Finland, Latvia and Lithuania. This system is based on nationally representative samples and self-administered mailed questionnaires. In comparing the results of national surveys, the awareness of the direction and socioeconomic patterning of the response bias is essential. METHODS The data were gathered from the cross-sectional surveys conducted in 1998 from Estonia (n = 1362), Finland (n = 3504), Latvia (n = 2322) and Lithuania (n = 1874). An analysis was made of the prevalence of late response, completeness of information obtained from respondents and the magnitude of response bias on the prevalence estimates of health behaviour indicators. RESULTS The response rates were comparatively high: 68% in Estonia, 70% in Finland, 77% in Latvia and 62% in Lithuania. Late response was weakly related to age, education or place of residence. The total proportion of missing information was below 10% and the sociodemographic patterning for this missing information was similar in all countries. Thus, older and less-educated respondents had more missing information on their questionnaires. Response bias of the prevalence estimates was minimal when it was calculated by using information obtained from late respondents. CONCLUSIONS The level of nonresponse and missing information was comparable in different countries, not information on health behaviour. Therefore special efforts are needed to design a questionnaire form which appears equally relevant to all respondent groups. The follow-up mailings were an effective way to increase the total response rate, but it was unlikely that they provided an effective way to reach the 'hard core' nonrespondents.