Life-expectancy in a nonhomogeneous population

Mortality differentials exist even among people of the same age and environmental risks. These differences are due to individual frailty (risks due to biological or behavioural reasons) not considered in the computation of conventional life-tables (where populations are assumed to be homogeneous). This note seeks to show empirically that life-expectancies computed from homogeneous population models are overstated for ages less than the mean, and are understated for higher ages. It is also shown that life-expectancy curves for both homogeneous and nonhomogeneous populations intersect at the mean survival age (when the most frail would have all died off) and at the maximum age lived in the population (when the last members of the cohort would have died off).