How long do we live? Demographic models and reflections on tempo effects: An introduction
The measurement of human longevity is one of the oldest topics in demography. The most widely used measure of longevity is the period life expectancy at birth which is calculated from age specific death rates by life table methods that originated with Gra
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ground The measurement of human longevity is one of the oldest topics in demography. The most widely used measure of longevity is the period life expectancy at birth which is calculated from age specific death rates by life table methods that originated with Graunt (1661) and have been standard in the field for well over a century. Period life expectancy equals the mean age at death in a synthetic cohort and it should be distinguished from the actual cohort life expectancies calculated for a group of individuals observed over long time periods. A tempo effect is defined as an inflation or deflation of the period incidence of a demographic event (e.g., births, marriages, deaths) resulting from a rise or fall in the mean age at which the event occurs (Bongaarts and Feeney, in this volume p.11 and p.29). The existence of tempo effects has been well established in measures of fertility and nuptiality but the idea that mortality measures may be also affected is new and controversial. Tempo effects were first discovered and analyzed in the study of fertility. If women shift the ages at which they bear children upward without changing their completed fertility, annual numbers of births will be less than they would have been because the same number of births will be spread out over a longer time period. Similarly, if women begin to have children at younger ages, annual numbers of births will be larger than they would have been because the same number of births occurs over a shorter time period. These changes in annual number of births induced by changes in the timing of childbearing are tempo effects. The post-war “baby boom” in the United States, for example, was due in part to a decline in the mean age at childbearing during the late 1940s and the 1950s (Ryder, 1964,1980) and in much of Europe recent period fertility levels are depressed by tempo effects resulting from the postponement of childbearing (Sobotka, 2004). Tempo effects complicate the study of
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Elisabetta Barbi
levels and trends of fertility because they produce changes in period fertility rates that depend on the rate at which the mean age at childbearing changes, independently of changes in completed fertility of cohorts. Ryder (1956) introduced the term “timing distortion” to refer to tempo effects in the total fertility rate because they are undesirable in most analyses of fertility levels and trends. Ryder’s pioneering work established the existence of tempo distortions in the total fertility rate, but he did not propose quantitative adjustments to remove tempo distortions. This may be explained in part by his strong emphasis on the conceptual priority of cohort fertility measures. Bongaarts and Feeney (1998) first proposed to remove tempo distortion from the period total fertility rate. Their tempo adjustment is obtained by dividing the observed total fertility rate by 1 − r, where r equals the annual change in the period mean age at birth. Recent applications of this method to obtain tempo adjusted fertility levels by birth order in many European countries are pre
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