Life expectancy is a statistical measure of the average (see below)
time an organism is expected to live, based on the year of its birth,
its current age, and other demographic factors including gender. The
most commonly used measure is life expectancy at birth (LEB), which
can be defined in two ways. Cohort LEB is the mean length of life of
an actual birth cohort (all individuals born in a given year) and can
be computed only for cohorts born many decades ago, so that all their
members have died. Period LEB is the mean length of life of a
hypothetical cohort assumed to be exposed, from birth through death,
to the mortality rates observed at a given year.National LEB figures
reported by national agencies and international organizations for
human populations are indeed estimates of period LEB. In the Bronze
Age and the Iron Age, human LEB was 26 years; the 2010 world LEB was
67.2 years. For recent years, LEB in Eswatini (Swaziland) is about 49,
while LEB in Japan is about 83. 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. For example, a society
with a LEB of 40 may have few people dying at precisely 40: most will
die before 30 or after 55. In populations 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. 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 5 would be another 65.[clarification needed] 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. However, pre-modern societies still had universally higher
mortality rates and universally lower life expectancies at every age
for both genders, and this example was relatively rare. In societies
with life expectancies of 30, for instance, a 40-year remaining
timespan at age 5 may not have been uncommon, but a 60-year one
was.Mathematically, life expectancy is the mean number of years of
life remaining at a given age, assuming age-specific mortality rates
remain at their most recently measured levels. It is denoted by e x
{\displaystyle e_{x}} ,[a] which means the mean number of subsequent
years of life for someone now aged x {\displaystyle x} , according to
a particular mortality experience. Longevity, maximum lifespan, and
life expectancy are not synonyms. Life expectancy is defined
statistically as the mean 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 lifespan of some members of a
population. Maximum lifespan is the age at death for the longest-lived
individual of a species. Moreover, because life expectancy is an
average, a particular person may die many years before or many years
after the "expected" survival. The term "maximum lifespan" has a quite
different meaning and is more related to longevity.
time an organism is expected to live, based on the year of its birth,
its current age, and other demographic factors including gender. The
most commonly used measure is life expectancy at birth (LEB), which
can be defined in two ways. Cohort LEB is the mean length of life of
an actual birth cohort (all individuals born in a given year) and can
be computed only for cohorts born many decades ago, so that all their
members have died. Period LEB is the mean length of life of a
hypothetical cohort assumed to be exposed, from birth through death,
to the mortality rates observed at a given year.National LEB figures
reported by national agencies and international organizations for
human populations are indeed estimates of period LEB. In the Bronze
Age and the Iron Age, human LEB was 26 years; the 2010 world LEB was
67.2 years. For recent years, LEB in Eswatini (Swaziland) is about 49,
while LEB in Japan is about 83. 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. For example, a society
with a LEB of 40 may have few people dying at precisely 40: most will
die before 30 or after 55. In populations 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. 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 5 would be another 65.[clarification needed] 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. However, pre-modern societies still had universally higher
mortality rates and universally lower life expectancies at every age
for both genders, and this example was relatively rare. In societies
with life expectancies of 30, for instance, a 40-year remaining
timespan at age 5 may not have been uncommon, but a 60-year one
was.Mathematically, life expectancy is the mean number of years of
life remaining at a given age, assuming age-specific mortality rates
remain at their most recently measured levels. It is denoted by e x
{\displaystyle e_{x}} ,[a] which means the mean number of subsequent
years of life for someone now aged x {\displaystyle x} , according to
a particular mortality experience. Longevity, maximum lifespan, and
life expectancy are not synonyms. Life expectancy is defined
statistically as the mean 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 lifespan of some members of a
population. Maximum lifespan is the age at death for the longest-lived
individual of a species. Moreover, because life expectancy is an
average, a particular person may die many years before or many years
after the "expected" survival. The term "maximum lifespan" has a quite
different meaning and is more related to longevity.
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