Variations in relative health inequalities: are they a mathematical artefact?

BACKGROUND: Substantial research has documented variations in the magnitude of relative socioeconomic differences in health across European countries, and within countries, across different age groups. The aim of this paper is to examine to what extent these variations are determined by differences in the overall rate or prevalence of a health outcome across countries and age-groups in the total population. METHODS: Three surveys (European Social Survey, and two different population census-mortality registry linked longitudinal data) were used. We plotted rates of mortality and prevalence of poor self-rated health against ratios of mortality and morbidity prevalence associated with educational level. We calculated Pearson coefficients to examine the magnitude of correlations. RESULTS: We found a significant negative correlation between total mortality rates and associated rate ratios of mortality by education in the SEDHA study (r = -0.40, p = 0.04), but not in the HUNT study (r = -0.37, p = 0.06). There was a weaker but significant negative correlation between the prevalence of poor health and associated prevalence ratios by education in the European social survey (r = -0.22, p = 0.00). Correlations increased as underlying prevalence and rates increased, while they were weaker or null at low prevalence or rates. CONCLUSION: We found some evidence that the magnitude of relative inequalities in mortality and morbidity is negatively correlated with underlying morbidity prevalence and mortality rates. Although correlations are moderate, underlying morbidity prevalence and mortality rates should be taken into account in the interpretation of variations in relative health inequalities among populations.