The Compression of Deaths Above the Mode

Roger A. Thatcher, Former Director of the Office of Population Censuses and Surveys, London
Siu Lan K Cheung, Hong Kong University of Science and Technology
Shiro Horiuchi, Rockefeller University
Jean Marie Robine, Institut National de la Santé et de la Recherche Médicale (INSERM)

The frequency distribution of ages at death has been shifting to the right, but it has not retained exactly the same shape. The ages of deaths above the mode have become more compressed. The present paper investigates the reasons for this phenomenon. One of the simple models of mortality is found to be appropriate and mathematically tractable. Changes in the modal age of death and in the compression are shown to be driven by the way in which age-specific death rates fall at ages 70 and over. Both can be predicted from the death rates. Results are illustrated by data from the English Life Tables and Interim Life Tables, and these are confirmed by extensive data for six countries. This paper also illustrates how the slope of the mortality curve can be steepened while people are living longer, thus implying that the traditional ageing rate is not a valid measure of senescence.