Life expectancy (LE) based on a period life table (PLT) traditionally serves as a general population summary metric. It is, however, becoming more frequently reported for chronically afflicted subpopulations.
In general populations, there is always an obvious real cohort sharing the hypothetical PLT cohort characteristics, and the LE estimate is intuitively understood as that real cohort mean survival time, assuming constancy of death risks. In diseased subpopulations, the correspondence between the hypothetical cohort and a real cohort is not straightforward. Furthermore, the excess mortality of chronic diseases usually changes according to age at onset and time since onset. The standard PLT method does not allow for proper control of these issues, so the LE estimate can only be deemed valid under specific assumptions.
Without clear statements about the real cohort to whom the estimate is intended and the assumptions allowing disregard of the effect of age at onset and time since onset, LEs of afflicted subpopulations computed with the PLT are only abstract numbers summarizing mortality rates. If called “life expectancy,” they can be seriously misleading. The same applies to health-adjusted LE.
- Linda Perron, Marc Simard, Denis Hamel, and Ernest Lo are with the Institut national de santé publique du Québec, Quebec City, QC. Linda Perron and Jacques Brisson are with the Département de médecine sociale et préventive, Faculté de médecine, Université Laval, Quebec City. Jacques Brisson is also with the Centre de recherche du chu de Québec-Université Laval, Centre hospitalier universitaire de Québec, Quebec City. Ernest Lo is also with the Department of Epidemiology, Biostatistics, and Occupational Health, McGill University, Montreal, QC.