A selective follow-up study on a public health survey

BACKGROUND The non-response rates in surveys are increasing which is problematic as it means that a progressively smaller proportion of the population represents the majority, and it is uncertain how health survey results are affected. This follow-up was performed on the non-responders to the postal questionnaire in the public health survey Life and Health, conducted in Örebro County Council, Sweden, where large differences in response rates had been found between different socio-demographic groups and geographical areas. The main objective was to analyse non-response bias regarding self-rated health. METHODS This follow-up study was conducted as a census to all non-responders in the area that had the lowest response rate and, in one other geographical area used as a control. It was carried out by telephone interviews, 49.3% (580 individuals) answered the follow-up. The outcome variable was self-rated health, a main variable in public health surveys. Differences in response patterns between responders and initial non-responders were approximated by prevalences with confidence intervals and adjusted odds ratios. RESULTS Poor health was more common in the initial non-response group than among the responders, even with consideration given to sex, age, country of birth and education. However, good health was equally common among responders and initial non-responders. CONCLUSIONS Public health surveys can be biased due to certain groups being under-represented or not represented at all. For this reason, in repeated public health surveys, we recommend selective follow-ups of such groups at regular intervals.