Background: Individuals of lower socio-economic status (SES) are less likely to participate in health surveys than individuals of a higher SES. It is, however, not known whether this difference in participation is associated with health status. This study sets out to assess whether a population health survey gives biased estimates of socio-economic inequalities in self-reported health.
Methods: We compared two independent cross-national data collections, a national health interview survey (n = 10,164) and a census (n = 8,491,528), both carried out in Belgium in 2001 and posing the same health question. We computed the prevalence ratios of poor subjective health among socio-economic groups. To estimate the bias, a relative odds ratio (ROR) was computed as the ratio of the survey prevalence ratio to the census prevalence ratio.
Results: Less-educated individuals had a lower risk of poor health status in the survey [Prevalence ratio = 1.66, 95% confidence interval (CI): 1.48-1.86] than in the census (Prevalence ratio = 2.23) leading to an underestimation of the risk associated with low education (ROR = 0.74, 95% CI 0.66-0.83). Compared with better-off groups, those who were not working or who were less educated were generally less likely to participate in the survey when they had a poor health status.
Conclusions: Overall, the health survey underestimated the effects of low SES on poor health status, due to selection bias. We conclude that strategies to improve participation among disadvantaged socio-economic groups should be identified.