Competing Effects on the Average Age of Infant Death

Abstract

Abstract In recent decades, the relationship between the average length of life for those who die in the first year of life—the life table quantity a10—and the level of infant mortality, on which its calculation is often based, has broken down. The very low levels of infant mortality in the developed world correspond to a range of a10 quantities. We illustrate the competing effect of falling mortality and reduction in preterm births on a10 through two populations with very different levels of premature birth—infants born to non-Hispanic White mothers and infants born to non-Hispanic Black mothers in the United States—using linked birth and infant death cohort data. Through simulation, we further demonstrate that falling mortality reduces a10, while a reduction in premature births increases it. We use these observations to motivate the formulation of a new approximation formula for a10 in low-mortality contexts, which aims to incorporate differences in preterm birth through a proxy measure—the ratio of infant to under-five mortality. Models are built and tested using data from the Human Mortality Database. Model results and validation show that the newly proposed model outperforms existing alternatives.