Removing the health domain from the Index of Multiple Deprivation 2004 - effect on measured inequalities in census measure of health

The Index of Multiple Deprivation (IMD) 2004 is a summary measure of area-level deprivation in England that combines weighted scores in seven deprivation domains. IMD 2004 is used extensively by local public health departments and researchers to describe and monitor socioeconomic inequalities in health. However, the inclusion of a health domain in IMD 2004 leads to the possibility of 'mathematical coupling' where a relationship between IMD 2004 and markers of health is predicated by the inclusion of health in IMD 2004-effectively placing measures of health on both sides of the correlation equation. We explored the effect of removing the health domain from IMD 2004 on assignment of small areas to deprivation groups and measured inequalities in health. There was excellent agreement between the deprivation quintiles that small areas were assigned to by IMD 2004 and IMD 2004-minus-health (kappa = 0.895). Removing the health domain had little, practical, effect on measured socioeconomic inequalities in census measures of health. These findings may not hold for other measures of health, and in the context of socioeconomic inequalities in health, removing the health domain from IMD 2004 probably represents best practice.