Relative Index of Inequality

The relative index of inequality (RII) is the relative counterpart of the slope index of inequality: instead of the absolute difference in a health outcome between the bottom and top of the socioeconomic hierarchy, it expresses that difference as a ratio. Like the SII, it is built from a regression of the outcome on each group's position in the cumulative socioeconomic distribution, so it uses the whole population and accounts for group sizes rather than comparing only the extreme categories. Mackenbach and Kunst's 1997 overview recommended the RII alongside the SII as the standard pair of summary measures for socioeconomic health inequality, precisely because relative and absolute inequality can move in opposite directions and both need to be reported. Sergeant and Firth's 2006 Biostatistics paper clarified the various definitions of the RII, compared estimation strategies, and supplied a parametric bootstrap for valid confidence intervals. The RII is dimensionless, which makes it directly comparable across outcomes, time periods, and populations with very different baseline rates. It is a mainstay of comparative health-inequality research and routine surveillance.