Life expectancy is a statistical measure of how long a person or organism may live, based on the year of their birth, their current age and other demographic factors including sex. At a given age, life expectancy is the average number of years that is likely to be lived by a group of individuals (of age ''x'') exposed to the same mortality conditions until they die. The most commonly used measure of life expectancy is life expectancy at age zero; that is, at birth (LEB), which can be defined in two ways: while ''cohort'' LEB is the mean length of life of an actual birth cohort (all individuals born a given year) and can be computed only for cohorts that were born many decades ago, so that all their members died, ''period'' LEB is the mean length of life of a hypothetical cohort assumed to be exposed since birth until death of all their members to the mortality rates observed at a given year.〔S. Shryok, J. S. Siegel et al. The Methods and Materials of Demography. Washington, DC, US Bureau of the Census, 1973〕
National LEB figures reported by statistical national agencies and international organizations are indeed estimates of ''period'' LEB.
In the Bronze and Iron Age LEB was 26 years; the 2010 world LEB was 67.2. For recent years in Swaziland LEB is about 49 years while in Japan is about 83 years. The combination of high infant mortality and deaths in young adulthood from accidents, epidemics, plagues, wars, and childbirth, particularly before modern medicine was widely available, significantly lowers LEB. But for those who survive early hazards, a life expectancy of sixty or seventy would not be uncommon. For example, a society with a LEB of 40 may have few people dying at age 40: most will die before 30 years of age or very few after 55. In countries with high infant mortality rates, LEB is highly sensitive to the rate of death in the first few years of life. Because of this sensitivity to infant mortality, LEB can be subjected to gross misinterpretation, leading one to believe that a population with a low LEB 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 at exactly 70 years old, LEB will be about 36 years, while about 25% 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 hypothetical population above, life expectancy at age 5 would be another 65 years. Aggregate population measures, such as the proportion of the population in various age groups, should also be used alongside individual-based measures like formal life expectancy when analyzing population structure and dynamics.
Mathematically, life expectancy is the mean number of years of life remaining at a given age, assuming constant mortality rates. It is denoted by , which means the average number of subsequent years of life for someone now aged , according to a particular mortality experience. Longevity and life expectancy are not synonyms. Life expectancy is defined statistically as the average number of years remaining for an individual or a group of people at a given age. Longevity refers to the characteristics of the relatively long life span of some members of a population. Moreover, because life expectancy is an average, a particular person may well die many years before or many years after their "expected" survival. The term "maximum life span" has a quite different meaning and is more related to longevity.
Life expectancy is also used in plant or animal ecology; 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 between failures" (MTBF) are used in engineering.
Human beings at birth are expected to live on average 49.42 years in Swaziland〔Url=https://www.cia.gov/library/publications/the-world-factbook/geos/wz.html〕 and 82.6 years in Japan, although Japan's recorded life expectancy may have been very slightly increased by counting many infant deaths as stillborn. An analysis published in 2011 in ''The Lancet'' attributes Japanese life expectancy to equal opportunities and public health as well as diet.
The oldest confirmed recorded age for any human is 122 years (see Jeanne Calment). 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.〔Santrock, John (2007). Life Expectancy. A Topical Approach to: Life-Span Development(pp. 128-132). New York, New York: The McGraw-Hill Companies, Inc.〕
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