- In health economics, a concentration index is a means of quantifying the degree of income-related inequality in health. Where there is no income-related inequality, the concentration index is zero. The concentration curve in the diagram shows the relationship between the cumulative percentage of the population or sample on the horizontal axis, ranked by income and beginning with the poorest on the left, and the cumulative percentage of ill-health (ranked by, say, self-reported health status) on the vertical axis beginning with the sickest at the bottom. The concentration index is defined as twice the area between the concentration curve and the line of equality (the 45o line running from the south-west corner to the north-east corner). The convention is that the index takes a negative value when the curve lies above the line of equality, indicating disproportionate concentration of ill-health among the poor, and a positive value when it lies below the line of equality.