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 < or = 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 > or = 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.