Non-response bias in a study of cardiovascular diseases, functional status and self-rated health among elderly men.

Objectives: To investigate to what extent differences in health status between respondents and drop-outs affected the associations between cardiovascular diseases and functional status and self-rated health in a population-based longitudinal health survey in elderly men.
Methods: During the 1993 survey of the Zutphen Elderly Study, a non-response survey was carried out. The prevalence of myocardial infarction and stroke, disabilities in basic activities of daily living (BADL) and mobility, and self-rated health were compared between non-respondents (n = 99) and respondents (n = 381). Associations between myocardial infarction and stroke on the one hand and functional status and self-rated health on the other were calculated for the total population and for the respondents to assess the amount of under- or overestimation of these associations.
Results: The health of non-respondents was worse than that of respondents in terms of stroke, disabilities in BADL and mobility and self-rated health. Due to this selective non-response, the associations between cardiovascular diseases and functional status and self-rated health were biased. Although most of the associations were slightly overestimated, the most important bias was the underestimation by 57% of the association between stroke and disabilities in BADL [total population: odds ratios (OR) = 6.1, 95% confidence interval (CI) = 2.7-13.9; respondents only: OR = 2.6, CI = 0.7-9.9].
Conclusion: Selective non-response might lead to bias in the prevalence of disease, disabilities and self-rated health as well as in the associations between disease and functional status and self-rated health. The direction and magnitude of this bias varies according to type of disease and health outcome and is therefore difficult to predict. The need to minimize non-response and to investigate its implications is recommended in every study.