Life expectancy



Life expectancy

The define of Life expectancy The arithmetic mean of the distribution of ages at death is called life expectancy (i.e., expectation of life at birth), and is widely used as an indicator of the health of the population.

It is consistently greater for females than for males, and the difference has widened over the old-year period between the two groups in population health states ,for example pregnancy and cancer cause of smoking ,age and sex,occubation…

A life table converts a set of age-specific mortality rates into a survival curve, from which summary statistics, such as life expectancy, can be derived. The procedure was developed first for humans, primarily for the purpose of calculating premiums for life insurance and annuities. Later the same approach was used to study the survival of patients, other living species, and inanimate objects.

Crude life tables were produced by the Roman Aemilius Macer in Rome in 225 C.E., and by John Graunt and William Petty in the seventeenth century. The astronomer Edmund Halley, in 1693, was the first to employ correct mathematical methods to calculate a life table, using vital statistics collated by Caspar Neumann of Breslau.

A life expectancy is easy to calculate if the mortality rates are known for each year of age. Starting with an arbitrary large number, for exp. say 1,000, of newborn infants, the number surviving to age one year can be estimated from the mortality rate in the first year of life. Then the number surviving to the age two years can be estimated using the mortality rate in the second year of life, and so on.

In practice, calculations are complicated by the fact that the mortality rates for single years of age cannot be estimated precisely, even in large populations, and some form of smoothing is required. The sharp decline in mortality during infancy and childhood, and the small numbers in extreme old age, also create problems. However the simple method can be used to calculate abridged life expectancy, using mortality rates for broader age groups, which are more readily available. Such tables are usually sufficient for public health purposes.

the distribution of the ages at death implied by the life tables of Figure. In the earlier table the distribution has two peaks, for Exp. one in the early childhood and the other in the 70–74 age group. In the later table the deaths in childhood have shifted to old age, producing a single peak at 80–84 years. The expectation of life at birth is forty-five years for eighty-one years for the life table.

The vast majority of published life tables are period life expectancy, which are based on mortality rates over a limited time period. Since mortality changes over time, no actual population experiences the survival depicted in a period life table. Such a table represents, instead, a hypothetical, or synthetic, cohort. Comparing values of life expectancy from different period life tables is really equivalent to comparing age-standardized mortality rates, since reciprocal of life expectancy is a form of age-standardized mortality rate.

True cohort, or generation, life tables require age-specific mortality rates covering nearly 100 years. Table 1 shows the life expectancy for two completed cohorts. Life expectancy is consistently greater for females than for males, and the difference has widened over the sixty-year period between the two groups. Information on the cause of death can be incorporated into the life table calculations in two ways. First, it is possible to calculate the number, out of those alive at a given age, who will die subsequently from a particular cause. Second, the gain in life expectancy which would be obtained by eliminating a particular cause of death can be calculated.

As life expectancy increases toward its natural upper limit, life tables become less useful as indices of the health of a population. Beginning in the 1960s procedures were developed to incorporate information on disability into life tables. The simplest approach is to multiply the number living at a

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Example :

Life Expectancy (Birth mortality )

at birth is defined as the average number of years that a newborn would live under mortality conditions prevailing at that time. For example, life expectancy for females born in the United States in 1900 was forty-nine years. This means that if mortality conditions existing in 1900 did not change, baby girls born at that time would have lived, on average, until they were forty-nine. In addition to life expectancy at birth, one can also examine life expectancy at other ages. For example, life expectancy at age sixty (which was fifteen years for women in 1900) is the average number of years of life remaining for someone who survives to age sixty, under mortality conditions prevailing at that time. A life table provides information on life expectancy at various ages. When correctly understood, life expectancy provides a useful summary measure of mortality conditions at a particular time in history.

Although life expectancy is a good starting point for discussing mortality patterns, it is important to note two significant limitations of this measure. First, mortality conditions often change over time, so this measure may not reflect the actual experience of a birth cohort. (A birth cohort consists of all individuals born in a particular time period.) To illustrate this point, females born in the United States in 1900 actually lived for an average of fifty-eight years. The discrepancy between life expectancy in 1900 and the average years lived by those born in 1900 occurred because mortality conditions improved as this cohort aged over the twentieth century. The second limitation of life expectancy as a mortality index is its failure to reveal anything about the distribution of deaths across ages. Relatively few of the girls born in 1900 actually died around age forty-nine; 20 percent died before reaching age ten, and over fifty percent were still alive at age seventy. In other words, the average age at death does not mean that this was the typical experience of individuals. Given the limited information contained in the life expectancy statistic, a satisfying discussion of changing mortality experiences in American history must use additional information on the timing and patterning of deaths.

To calculate the life expectancy for a population, one would ideally have a complete registration of deaths by age and a complete enumeration of the population by age. With these data, it is a straightforward exercise to calculate age-specific death rates and to construct the life table. In the United States, mortality and population data of good quality are available for most of the twentieth century, so we can report with confidence life expectancy patterns over this period. Because of data limitations, there is less certainty about mortality conditions in earlier American history. However, a number of careful and creative studies of the existing death records for some communities (or other populations) provide enough information to justify a discussion of changing mortality conditions from the colonial era to the present.

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