Survey error in measuring socio-economic risk factors of health status: a comparison of a survey and a census

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 491528), 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.