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	Concentration Index	Slope Index of Inequality
Field	Social Epidemiology	Social Epidemiology
Family	Process / pipeline	Regression model
Year of origin	1991	1997
Originator	Adam Wagstaff, Pierfilippo Paci & Eddy van Doorslaer; Nanak Kakwani	Anton E. Kunst & Johan P. Mackenbach; Adam Wagstaff et al.
Type	Rank-based summary index of socioeconomic inequality in health	Regression-based absolute measure of health inequality across ordered SES groups
Seminal source	Wagstaff, A., Paci, P., & van Doorslaer, E. (1991). On the measurement of inequalities in health. Social Science & Medicine, 33(5), 545-557.	Mackenbach, J. P., & Kunst, A. E. (1997). Measuring the magnitude of socio-economic inequalities in health: an overview of available measures illustrated with two examples from Europe. Social Science & Medicine, 44(6), 757-771.
Aliases	Health Concentration Index, Concentration Curve and Index, Wagstaff Concentration Index, Erreygers Corrected Concentration Index	SII, Slope Index, Absolute Slope Index of Inequality, Kunst-Mackenbach Slope Index
Related	4	4
Summary	The concentration index is the standard summary measure of socioeconomic inequality in health: it quantifies the degree to which a health outcome - illness, mortality, malnutrition, or healthcare use - is disproportionately concentrated among the poor or the rich. It is built from the concentration curve, which plots the cumulative share of health against the cumulative share of the population ranked from poorest to richest, and the index is simply twice the area between that curve and the line of perfect equality. Wagstaff, Paci, and van Doorslaer's 1991 critique of inequality measures argued that, unlike the simple range or the Gini, the concentration index properly reflects the socioeconomic dimension of health inequality and the experience of the whole distribution. Kakwani, Wagstaff, and van Doorslaer's 1997 paper then supplied a computational formula, a convenient regression estimator, and the asymptotic variance needed for statistical inference. The index ranges from minus one to plus one, with zero meaning no socioeconomic gradient, a negative value meaning ill health concentrates among the poor, and a positive value the reverse. It has become the lingua franca of health-equity monitoring at agencies like the World Bank and WHO.	The slope index of inequality (SII) is a regression-based summary measure that expresses the absolute difference in a health outcome between the bottom and the top of the socioeconomic hierarchy. Rather than comparing only the most extreme groups - which discards information and is sensitive to how categories are defined - it regresses the outcome on each group's relative position in the cumulative socioeconomic distribution and reads the inequality off the fitted line. Mackenbach and Kunst's 1997 Social Science & Medicine overview made the SII, together with its relative counterpart, the recommended pair of measures for quantifying socioeconomic inequalities in health because they use the whole population and account for group sizes. The SII is measured in the natural units of the outcome - extra deaths per 100,000, additional percentage points of disease prevalence - which makes it directly meaningful for public-health and policy audiences. Wagstaff, Paci, and van Doorslaer had earlier argued that such regression-on-rank measures, alongside the concentration index, are among the few that properly reflect the socioeconomic dimension of health. The SII has become a standard tool in health-inequality monitoring across Europe and beyond.