Some Methodological Remarks on Self-Rated Health

Objectives: In analyses concerned with self-rated health it is common to dichotomise an ordinal-scale health measure and compare different subgroups of a population on basis of odds ratios from logistic regression models. Selfrated health is often explored also in wide age intervals. Since people s health correlates strongly with their age, that approach is empirically problematic, particularly when it comes to determining the cut-off point for dichotomisation and the role of age-dependent covariates. We set out to investigate these issues, and prove them to be practically relevant. Study design: Using a highly representative data set, the Health 2000 survey in Finland, we focus on ages 35-64 years. Separate analyses are undertaken for this whole age interval, and for three shorter age intervals. Methods: Self-rated health was in the survey measured on a five-point ordinal scale. We dichotomised the responses in two alternative ways (bad health categorised as poor or fairly poor , and as poor , fairly poor or average ), and explored the estimated effects of some standard covariates. Results: When the whole age interval was analysed, the choice of cut-off point for health dichotomisation had only a modest impact on the estimated effects of the covariates. However, with a narrower categorisation of poor health, the effect of educational level, as well as of marital status, was found to be highly age-dependent. Conclusions: Researchers and health policy practitioners should be aware of the risks for drawing misleading or even incorrect conclusions from studies of self-rated health based on wide age intervals that do not explicitly account for agedependent covariates.