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( Life expectancy)
Life expectancy is the expected (in the statistical sense) number of years of life remaining at a given age.[1] It is denoted by ex, 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, ie excluding fractions of a year. The corresponding statistic including fractions of a year, ie 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. Life expectancy is usually calculated separately for males and females. 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. 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. The term life expectancy is most often used in the context of human populations, but is also used in plant or animal ecology[2]; 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[3] although the related term shelf life is used for consumer products and the term mean time to breakdown (MTTB) is used in engineering literature. Humans live on average 39.5 years in Swaziland[4] and 81 years in Japan (2008 est.), although Japan's recorded life expectancy may have been very slightly increased by counting many infant deaths as stillborn.[5] The oldest confirmed recorded age for any human is 122 years (see Jeanne Calment), though some people are reported to have lived longer. This is referred to as the "maximum life span", which is the upper boundary of life, the maximum number of years any human is known to have lived.[6]
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