Life Tables in the Measurement of Mortality and Longevity

Abstract

THE LIFE table, which provides detailed information on survival, has numerous applications. In public health, the life table may be used in assessing the health status, health trends, and health needs of communities. In addition, the life table is a basic tool in demography and is frequently used in estimating, projecting, and analyzing population changes. The data contained in the life table may also be applied in planning services for the population, such as prediction of markets, location of schools, extension of pipelines, and construction of hospitals. By providing statis-tical values of expectation of life and probability of death and survivorship, the life table performs two basic functions. Not only does it summarize the mortality experience of a selected population group during a specific calendar period, but it makes possible the comparison of mortality experiences in two or more population groups. Such comparisons may be of two types: on the one hand, it is possible to compare the mortality experiences of a particular population group during two, or more, calendar periods, for example, U.S. white males in 1900 and 1958; on the other hand, two, or more, different population groups may be contrasted on the basis of their mortality experiences during the same calendar period, for example, U.S. white males and white females in 1958. Two types of life tables-the generation life table and the current life table-should be distinguished. The former provides a longitudinal perspective by following an actual generation of births, or a cohort, from the moment of birth through consecutive ages in successive calendar years. Based on the age-specific mortality rates of the cohort, which are observed during individual calendar years of exposure, the generation life table reflects actual mortality experience of the cohort throughout its lifetime. This article is concerned only with the better known current life table, which is built upon the assumption of a hypothetical cohort, born during a particular calendar period and subject to the age-specific mortality rates observed in an actual population during the same period. Thus, for example, a life table for 1958 assumes a hypothetical cohort born in 1958 to be subject throughout its lifetime to the identical agespecific mortality rates prevailing in the actual population in 1958. No allowances are made for year-to-year changes in age-specific mortality as the hypothetical cohort passes through life. In the life table, except for the purpose of computing mortality rates, the age distribution of the actual population is disregarded. No provision is made for such factors as migration and fertility, which affect age distribution in an actual population. These stipulations are desirable from the point of view of comparing mortality experiences in two or more population groups. Elimination of factors other than mortality keeps to a minimum the number of variables which may limit comparability of mortality experiences.