http://whqlibdoc.who.int/hq/1999/a68869.pdf

Health inequalities are prominent in the policy agenda. Average achievement is no longer considered a sufficient indicator of a country'sperformance on health; rather, the distribution of health in the population is also key. The World Health Organization (WHO) is interested in measuring health inequality as a distinct dimension of the performance of health systems. We define health inequality to be variations in health status across individuals in a population. This approach, which is consistent with the measurement of inequality in other fields, such as economics, allows us to perform cross-country comparisons and study the determinants of health inequality.