Life Expectancy

Life expectancy is the expected (in the statistical sense) number of years of life remaining at a given age. It is denoted by e_x, which means the average number of subsequent years of life for someone now aged x, according to a particular mortality experience. (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.) In the modern era, life expectancy has increased so dramatically as to call into question the ability to use current data for very long range predictions into the future.

The term that is known as life expectancy is most often used in the context of human populations, but is also used in plant or animal ecology; 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 although the related term shelf life is used for consumer products and the terms "mean time to breakdown" (MTTB) and "mean time before failures" (MTBF) are used in engineering l.a.