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	Poverty Gap Index	Watts Poverty Index
Field	Economics	Economics
Family	Process / pipeline	Process / pipeline
Year of origin	1984	1968
Originator	James Foster, Joel Greer & Erik Thorbecke (FGT alpha = 1)	Harold W. Watts (1968); axiomatized by Buhong Zheng (1993)
Type	Money-metric poverty depth measure	Distribution-sensitive poverty measure
Seminal source	Foster, J., Greer, J., & Thorbecke, E. (1984). A class of decomposable poverty measures. Econometrica, 52(3), 761–766.	Zheng, B. (1993). An axiomatic characterization of the Watts poverty index. Economics Letters, 42(4), 347–353.
Aliases	Poverty Gap Ratio, Income Gap Measure, Mean Normalized Shortfall, Depth of Poverty	Watts Index, Watts Poverty Measure, Log Shortfall Poverty Index
Related	3	3
Summary	The poverty gap index is the member of the Foster-Greer-Thorbecke family at alpha = 1 and the standard money-metric measure of the depth of poverty. Where the headcount ratio merely counts who is poor, the poverty gap averages how far the poor fall below the poverty line, expressed as a fraction of that line and spread over the whole population. It can be read as the per-capita resource shortfall — the share of the poverty line, per person, that perfect targeting would need to transfer to eliminate poverty — making it the natural complement to the headcount when judging the cost and intensity of poverty.	The Watts index, proposed by Harold Watts in 1968, was the first poverty measure to be sensitive to the distribution of income among the poor, anticipating the axiomatic poverty-measurement literature by nearly a decade. It averages, over the whole population, the natural logarithm of the ratio of the poverty line to each poor person's income. Because the log gives ever-larger weight to incomes near zero, the Watts index satisfies the strong transfer principles that the headcount and the linear poverty gap fail, and Buhong Zheng's 1993 axiomatic characterization established it as the smallest distribution-sensitive measure satisfying a natural set of axioms.