Life expectancy: what does it measure?

Abstract

Life expectancy (LE) is considered a straightforward summary measure of mortality that comes with an implicit age standardisation. Thus, it has become common to present differences in mortality across populations as differences in LE, instead of, say, relative risks. However, most of the time LE does not quite provide what the term promises. LE is based on a synthetic cohort and is therefore not the true LE of anyone. Also, the implicit age standardisation is construed in such a way that it can be questioned whether it standardises age at all. In this paper, we examine LE from the point of view of its applicability to epidemiological and public health research and provide examples on the relation between an LE difference and a relative risk. We argue that the age standardisation in estimations of LE is not straightforward since it is standardised against different age distributions and that the translation of changes in age specific mortality into change in remaining LE will depend on the level and the distribution of mortality in the population. We conclude that LE is not the measure of choice in aetiological research or in research with the aim to identify risk factors of death, but that LE may be a compelling choice in public health contexts. One cannot escape the thought that the mathematical elegance of LE has contributed to its popularity.

Keywords: epidemiology; public health; statistics & research methods.

Figure 1

Death rates (A) and survival curves (B) for women in the USA in 2013 with and without cancer. Death rates were estimated using death counts and corresponding person-years, which have been downloaded from the Human Mortality Database. Cancer deaths were extracted using International Classification of Diseases-10th edition codes B00-D48 from the Mortality Multiple Cause File for the year 2013 provided in the website of the Centers for Disease Control and Prevention. The cause-eliminated life table has been estimated using standard methodology as outlined in Preston et al.

Figure 2

Gains in life expectancy (LE) at birth (A) and at age 50 (B) for different values of relative risks under five different levels of LE. Calculations are based on Swedish life tables for women and men combined for the 5-year period 2010–2014 (to avoid random fluctuations due to sampling variability). The data were downloaded on 9 July 2018, from the Human Mortality Database. The death rates were adjusted to the desired level of LE at birth (70, 75, 80, 85, 90) or remaining LE at age 50 (25, 30, 35, 40, 45) by shifting the age trajectory of death rates up or down. Next, the death rates were multiplied with factors ranging from 0 to 3 (with increments of 0.01) and estimated the corresponding differences in LE to the baseline mortality risk (relative risk=1). All life tables estimates are based on standard methodology.