Volume 13 - Article 11 | Pages 231-280

On the relationship between period and cohort mortality

By John R. Wilmoth

Print this page  Facebook  Twitter


Date received:23 May 2005
Date published:17 Nov 2005
Word count:9614
Keywords:life expectancy, life span, mortality, tempo-adjusted life expectancy


In this paper I explore the formal relationship between period and cohort mortality, focusing on a comparison of measures of mean lifespan. I consider not only the usual measures (life expectancy at birth for time periods and birth cohorts) but also some alternative measures that have been proposed recently.
I examine (and reject) the claim made by Bongaarts and Feeney that the level of period is distorted, or biased, due to changes in the timing of mortality. I show that their proposed alternative measure, called “tempo-adjusted” life expectancy, is exactly equivalent in its generalized form to a measure proposed by both Brouard and Guillot, the cross-sectional
average length of life (or CAL), which substitutes cohort survival probabilities for their period counterparts in the calculation of mean lifespan.
I conclude that this measure does not in any sense correct for a distortion in period life expectancy at birth, but rather offers an alternative measure of mean lifespan that is approximately equal to two analytically interesting quantities: 1) the mean age at death in a given year for a hypothetical population subject to observed historical mortality conditions but with a constant annual number of births; and 2) the mean age at death, , for a cohort born years ago.
However, I also observe that the trend in period does indeed offer a biased depiction of the pace of change in mean lifespan from cohort to cohort. Holding other factors constant, an historical increase in life expectancy at birth is somewhat faster when viewed from the perspective of cohorts (i.e., year of birth) than from the perspective of periods (i.e., year of death).

Author's Affiliation

John R. Wilmoth - University of California, Berkeley, United States of America [Email]

Other articles by the same author/authors in Demographic Research

» Introduction to the Special Collection “Human Mortality over Age, Time, Sex, and Place: The 1st HMD Symposium”
Volume 13 - Article 10

» The Cancer Transition in Japan since 1951
Volume 7 - Article 5

Most recent similar articles in Demographic Research

» The relative tail of longevity and the mean remaining lifetime
Volume 14 - Article 7    | Keywords: life expectancy, life span, mortality

» Contribution of smoking-attributable mortality to life-expectancy differences by marital status among Finnish men and women, 1971-2010
Volume 36 - Article 8    | Keywords: life expectancy, mortality

» Variance models of the last age interval and their impact on life expectancy at subnational scales
Volume 35 - Article 15    | Keywords: life expectancy, mortality

» Divergence without decoupling: Male and female life expectancy usually co-move
Volume 31 - Article 51    | Keywords: life expectancy, mortality

» Why the racial gap in life expectancy is declining in the United States
Volume 31 - Article 32    | Keywords: life expectancy, mortality


»Volume 13




Similar Articles



Jump to Article

Volume Page
Volume Article ID