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 figure shows the cumulative percentage of the population or sample on the horizontal axis sample, ranked by income, beginning with the poorest and the cumulative percentage of ill-health (say, fractions of deaths in a period) on the vertical axis. The concentration index is defined as twice the area between the concentration curve and the line of equality (the 45° line running
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00 100 Cumulative % of persons, by income from the south west corner to the north east). 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. Cf. Lorenz Curve.
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