Background: Selection bias in observational epidemiology – the notion that people who participate in a study are fundamentally different to those that 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), attempts were made to resurvey participants (HALS2). In 6484 men and women who took part in both surveys, we compared 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 and participants.
Results: Resurvey non-participants were, based on data from HALS-1, younger, of lower social class, had a lower prevalence of hypertension or self-reported limiting long-standing illness, but a higher prevalence of psychological distress (p-values<=0.027). The risk of death from future CVD was significantly higher in those that did not participate in HALS-2. However, the magnitude of the association between a series of risk factors and CVD mortality was essentially the same in the HALS-2 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 cardiovascular disease mortality. Future studies should examine the impact of non-response to baseline surveys on these relationships.