User:Forkandwait/life expectancy
It is denoted by ex, which means the average number of subsequent years of life for a hypothetical member of a population currently aged x, according to the mortality experience of the population to which he or she belongs. (In technical literature, this symbol means the average number of complete years of life remaining, excluding fractions of a year. The corresponding statistic including fractions of a year, the normal meaning of life expectancy, has a symbol with a small circle over the e.) The life expectancy of a group of individuals is heavily dependent on the criteria used to select the group, as different populations have widely different mortality rates. Life expectancy is usually calculated separately for males and females. Females live longer than males in countries with modern obstetric care.
In countries with high infant mortality rates, the life expectancy at birth is highly sensitive to the rate of death in the first few years of life. Because of this sensitivity to infant mortality, simple life expectancy at age zero can be subject to gross misinterpretation, leading one to believe that a population with a low overall life expectancy will necessarily have a small proportion of older people. For example, in a hypothetical stationary population in which half the population dies before the age of five, but everybody else dies exactly at 70 years old, the life expectancy at age zero will be about 35 years, while about 28% of the population will be between the ages of 50 and 70. Another measure such as life expectancy at age 5 (e5) can be used to exclude the effect of infant mortality to provide a simple measure of overall mortality rates other than in early childhood -- in the example population, life expectancy at age 5 would be 70 years. Aggregate population measures such as the proportion of the population in various age classes also complement individual-based measures like formal life expectancy when analyzing population structure and dynamics.
The term life expectancy is most often used in the context of human populations, but is also used in plant or animal ecology[1]; it is calculated by the analysis of life tables (also known as actuarial tables). The term life expectancy may also be used in the context of manufactured objects[2] although the related term shelf life is used for consumer products and the term "mean time to breakdown" (MTTB) is used in engineering literature.