Compare methods

	Slope Index of Inequality	Theil Index for Health Inequality
Field	Social Epidemiology	Social Epidemiology
Family	Regression model	Process / pipeline
Year of origin	1997	2008
Originator	Anton E. Kunst & Johan P. Mackenbach; Adam Wagstaff et al.	Henri Theil (entropy index); Sam Harper & John Lynch; Luisa Borrell & Makram Talih (health-disparity applications)
Type	Regression-based absolute measure of health inequality across ordered SES groups	Entropy-based decomposable index of inequality applied to health disparities
Seminal source	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.	Harper, S., Lynch, J., Meersman, S. C., Breen, N., Davis, W. W., & Reichman, M. E. (2008). An Overview of Methods for Monitoring Social Disparities in Cancer with an Example Using Trends in Lung Cancer Incidence by Area-Socioeconomic Position and Race-Ethnicity, 1992-2004. American Journal of Epidemiology, 167(8), 889-899.
Aliases	SII, Slope Index, Absolute Slope Index of Inequality, Kunst-Mackenbach Slope Index	Theil Index, Theil's T, Theil Entropy Index of Health Disparity, Symmetrized Theil Index
Related	4	4
Summary	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.	The Theil index is an entropy-based measure of inequality, adapted from information theory, that quantifies how disproportionately a quantity is distributed across groups - and in social epidemiology it measures how unequally ill health (or its burden) is shared across population subgroups defined by race-ethnicity, area socioeconomic position, or other categories. Its defining advantage over simpler disparity measures is additive decomposability: total inequality splits cleanly into a between-group component and a within-group component, so analysts can ask how much overall health disparity is due to differences among, say, racial groups versus differences among areas inside those groups. Harper, Lynch, and colleagues' 2008 American Journal of Epidemiology overview placed the Theil index among the recommended summary measures for monitoring social disparities in health, illustrating it with trends in U.S. lung-cancer incidence. Borrell and Talih's 2011 Statistics in Medicine paper introduced a symmetrized version that removes the standard index's dependence on which group is treated as the reference and derived its variance for complex survey data. Because it is reference-free, decomposable, and population-weighted, the Theil index is well suited to multi-group, nested disparity analysis where ordered-rank measures like the concentration index do not apply.