Impact of resurvey non-response on the associations between baseline risk factors and cardiovascular disease mortality: prospective cohort study

Background: Selection bias in observational epidemiology-the notion that people who participate in a study are fundamentally different from those who do not-is a perennial concern. In cohort studies, a potentially important but little investigated manifestation of selection bias is the distortion of the exposure-disease relationship according to participation status. Methods: Seven years after the original UK Health and Lifestyle Survey (HALS1; N = 6484), attempts were made to resurvey participants (HALS2). The baseline characteristics, mortality experience following the completion of HALS2 and, finally, the baseline risk factor cardiovascular disease (CVD) mortality gradients in HALS2 non-participants (N = 1894) and participants (N = 4590) were compared. Results: Resurvey non-participants, based on data from HALS1, were younger, were of lower social class and had a lower prevalence of hypertension or self-reported limiting long-standing illness, but a higher prevalence of psychological distress (p <= 0.027). The risk of death from future CVD was significantly higher in those baseline study members who did not participate in HALS2. However, the magnitude of the association between a series of risk factors and CVD mortality was essentially the same in the HALS2 non-participants and participants (p value for interaction >= 0.108). Conclusion: In the present cohort study, non-response at resurvey did not bias the observed associations between baseline risk factors and later CVD mortality. Future studies should also examine the impact of non-response to baseline surveys on these relationships.