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	Keyfitz Entropy	Lifespan Inequality
分野	人口学	人口学
系統	Process / pipeline	Process / pipeline
提唱年	1977	2003
提唱者	Nathan Keyfitz	Lifespan-variation literature; life disparity formalized by Vaupel & Canudas-Romo
種類	Elasticity of life expectancy to proportional mortality change / lifespan dispersion measure	Measures of variability in the age-at-death distribution
原典	Keyfitz, N. (1977). Applied Mathematical Demography. John Wiley & Sons, New York. ISBN: 9780471473503	Vaupel, J. W., & Canudas-Romo, V. (2003). Decomposing change in life expectancy: A bouquet of formulas in honor of Nathan Keyfitz's 90th birthday. Demography, 40(2), 201–216.
別名	Life-Table Entropy, Keyfitz-Leser Entropy, Entropy of the Survival Curve	Lifespan Variation, Life Disparity, Variation in Age at Death
関連	4	4
概要	Keyfitz's entropy, usually written H, is a dimensionless summary of a life table that measures how sensitive life expectancy is to a proportional change in mortality, and equivalently how unequal the distribution of ages at death is. Introduced by Nathan Keyfitz, it is the elasticity of life expectancy at birth with respect to the force of mortality: an H near one means deaths are spread across all ages so that reducing mortality everywhere lengthens life proportionally, while an H near zero means deaths are concentrated near the maximum lifespan so further mortality reductions yield little gain. It bridges the demography of survival and the broader study of lifespan inequality.	Lifespan inequality measures how unequally length of life is distributed within a population — the spread of the life-table ages at death, not just their average. Two populations can share the same life expectancy yet differ sharply in how predictable death is: in one nearly everyone reaches old age, in the other deaths are scattered across all ages. A family of measures — life disparity (e†), the standard deviation of age at death, the life-table Gini coefficient, and Keyfitz entropy — quantifies this dispersion, complementing life expectancy with a measure of how fairly survival is shared.