National Center for Health Statistics

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Guide to tables in section 6
Death rates for a specific period may be summarized by the life table method to obtain measures of comparative longevity. There are two types of life tables-the generation or cohort life  table and the current life table. The generation life table provides a ''longitudinal'' perspective in that it follows the mortality experience of a particular cohort, all persons born in the year 1900, for example, from the moment of birth through consecutive ages in successive calendar years. Based on age-specific death rates observed through consecutive calendar years, the generation life table reflects the mortality experience of an actual cohort from birth until no lives remain in the group. To prepare just a single complete generation life table requires data over many years. It is not feasible to construct generation life tables entirely on the basis of actual data for cohorts born in this century (1). It is necessary to project data for the incomplete period for cohorts whose life spans are not yet complete (2).
The better-known current life table may, in contrast, be characterized as ''cross-sectional.' ' Unlike the generation life  table, the current life table does not represent the mortality  experience of an actual cohort. Rather, the current life table  considers a hypothetical cohort and assumes that it is subject to the age-specific death rates observed for an actual population during a particular period. Thus, for example, a current life table for 1993 assumes a hypothetical cohort subject throughout its lifetime to the age-specific death rates prevailing for the actual population in 1993. The current life table may thus be characterized as rendering a ''snapshot'' of current mortality experience, and shows the long-range implications of a set of agespecific death rates that prevailed in a given year. In this section the term ''life table'' refers only to the current life table and  Beginning with 1958, provisional abridged life tables have been published, for the total population only, in the ''Annual Summary of Births, Marriages, Divorces, and Deaths, United States,'' Monthly Vital Statistics Report; unpublished provisional life table data by race and sex are also produced annually. Values in these life tables are based on population estimates provided by the U.S. Bureau of the Census and on the estimated number of deaths derived from the Current Mortality Sample (CMS). The CMS consists of one-tenth of the death certificates filed in the vital statistics registration offices of each State, the District of Columbia, and New York City. The sample is taken by selecting 1 of every 10 death certificates received between 2 dates a month apart, regardless of the month or year in which the death occurred.

Life table values
The data used to prepare the abridged U.S. life tables for 1993 are the final mortality statistics and the July 1, 1993, population estimates by age, race, and sex prepared by the U. Life expectancy at birth for 1993 for the total population was 75.5 years. This represents the average number of years that the members of the life table cohort may expect to live at the time of birth (tables A and 6-1).
Survivors to specified ages-Another way of assessing the longevity of the life table cohort is by determining the proportion who survive to specified ages. The l x column provides the data for computing the proportion. For instance, for the total population 79,945 out of the original 1993 life table cohort of 100,000 (or 79.9 percent) were alive at exact age 65 (tables B and 6-2).
Median length of life-In addition to determining the proportion alive at a specified age, one can also compute the median age at death, the age at which exactly half the cohort (50,000 persons) still remain alive and half have died. For example, in 1993 the median age at death for the total population was 79.3 years (table B).

Trends and comparisons
In 1993 the average expectation of life at birth was 75.5 years, a decline of 0.3 years from the record high of 75.8 years in 1992. This was the first decrease in life expectancy at birth in the United States since 1980.
The expectation of life at birth for 1993 represents the average number of years that a group of infants born in 1993 would expect to live if the infants were to experience throughout their life the age-specific death rates prevailing in 1993. In 1993 life expectancy at birth for females was 78.8 years compared with 72.2 years for males; both figures represent decreases over 1992. The difference in life expectancy between the sexes was 6.6 years in 1993, smaller than the difference of 6.8 years in 1992. Historically, the difference in life expectancy between the sexes widened from 1900 to 1979 (2.0 years in 1900 and 7.8 years in 1975 and 1979), narrowed between 1979 and 1986 (7.8 years in 1979 to 7.0 years in 1986), and has subsequently fluctuated between 6.8 and 7.0 years. The difference of 6.6 years for 1993 is the narrowest the difference between the sexes has been since 1962.
Between 1992 and 1993, life expectancy for the white population decreased from the record high of 76.5 years to 76.3 years, and for the black population, from the record high of 69.6 years to 69.2 years. Although the difference in life expectancy between the black and white population narrowed from 7.6 Life expectancy differences between males and females widened for many years after the beginning of the century, but recently the differences have narrowed for the white population and have widened for the black population (table D). For the white population the difference between males and females increased from 2.9 years in 1900-1902 to 7.6 years by 1969-71; the difference has narrowed to 6.4 years for 1993, the lowest difference since 1956. For the black population, the difference in life expectancy between males and females increased from 2.5 years in 1900-1902 to 8.8 years by 1979-81; it narrowed to 8.3 years by 1984; increased to 9.2 years by 1991, and was 9.1 years for 1993.
Life expectancy differences between the white and black populations have widened for males from a low of 6.4 years for 1982 and 1983 to 8.5 for 1993, the greatest difference during the period 1970-93. For females, the life expectancy differences between the white and black populations have widened from a low of 5.1 years for 1982 and 1984 to 5.8 years for 1993.
The 1993 life table may be used to compare life expectancies at any age from birth onward. For example, a person who has reached age 65 years may look forward to living to an older age, on the average, than one who has reached 50 years. On the basis of mortality experienced in 1993, a person aged 50 years could expect to live an average of 29.2 more years for a total of 79.2 years, and a person aged 65 years could expect to live an average of 17.3 more years for a total of 82.3 years (tables 6-1 and A). New Jersey data, 1962-64-The life tables for 1962 and 1963 for the six population groups involving race do not include data from New Jersey. This State omitted the item on race from its certificates of live birth, death, and fetal death in use at the beginning of 1962. The item was restored during the latter part of 1962. However, the certificate revision without this item was used for most of 1962 as well as for 1963. For computing vital rates, populations by age, race, and sex (excluding New Jersey) were estimated to obtain comparable denominators. Approximately 7 percent of the New Jersey death records for 1964 did not contain the race designation. When the records were being electronically processed for this State, the ''race not stated'' deaths were allocated to white or to black.  expectancy at birth in years, by race and sex: United States, 1900-1902 to 1969-71,  1969-71 to 1979-81, 1979-81 to 1984, 1984 to 1985, 1985 to 1986, 1986 to 1987, 1987 to 1988, 1988 to 1989, 1989 to 1990,  1990 to 1991, 1991 to 1992, and 1992 1990 to 1991. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1988 to 1989. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Nonresidents-Beginning in 1970 the deaths of nonresidents of the United States have been excluded from the life table statistics.
Estimates for single calendar years-Annual abridged life tables were initiated in 1945 for white males, white females, all other males, and all other females. The figures in table 6-5 by race and sex for the following years were estimated using a procedure other than the abridged life table methodology (5).

Population bases for computing life tables
The population used for computing life table values shown in this section (furnished by the U.S. Bureau of the Census) represents the resident population of the United States. The populations used for computing the 1993 life table values are based on the July 1, 1993, population estimates that are consistent with the 1990 census (6). The 1990 census counts by race and age were modified. Race was modified to be consistent with the Office of Management and Budget categories and historical categories for mortality data. The modification procedures for race and age are described in a census report (7).

Explanation of the columns of the life tables
Column 1-Age interval (x to x + n)-This column shows the age interval between the two exact ages indicated. For instance, ''20-25'' means the 5-year interval between the 20th and 25th birthdays.
Column 2-Proportion dying ( n q x )-This column shows the proportion of the cohort who are alive at the beginning of an indicated age interval and who will die before reaching the end of that age interval. For example, for males in the age interval 20-25 years, the proportion dying is 0.00822: Out of every 1,000 males alive and exactly 20 years of age at the beginning of the period, about 8 will die before reaching their 25th birthday. In other words, the n q x values represent probabilities that persons who are alive at the beginning of a specific age interval will die before reaching the beginning of the next age interval. The ''proportion dying'' column forms the basis of the life table. The life table is so constructed that all other columns are derived from it.
Column 3-Number surviving (l x )-This column shows the number of persons, starting with a cohort of 100,000 live births, who survive to the exact age marking the beginning of each age interval. The l x values are computed from the n q x values, which are successively applied to the remainder of the original 100,000 persons still alive at the beginning of each age interval. Thus out of 100,000 male babies born alive, 99,077 will complete the first year of life and enter the second; 98,857 will begin the sixth year; 97,997 will reach age 20; and 22,983 will live to age 85.
Column 4-Number dying ( n d x )-This column shows the number dying in each successive age interval out of 100,000 live births. Out of 100,000 males born alive, 923 will die in the first year of life; 193 in the succeeding 4 years; 806 in the 5-year period between exact ages 20 and 25, and 22,983 will die after reaching age 85. Each figure in column 4 is the difference between two successive figures in column 3.
Columns 5 and 6-Stationary population ( n L x and T x )-Suppose that a group of 100,000 individuals like that assumed in columns 3 and 4 is born every year and that the proportions dying in each such group in each age interval throughout the lives of the members are exactly those shown in column 2. If there were no migration and if the births were evenly distributed over the calendar year, the survivors of these births would make up what is called a stationary population-stationary because in such a population the number of persons living in any given age group would never change. When individuals left the group, either by death or by growing older and entering the next higher age group, their places would immediately be taken by persons entering from the next lower age group. Thus a census taken at any time in such a stationary community would always show the same total population and the same numerical distribution of that population among the various age groups. In such a stationary population supported by 100,000 annual births, column 3 shows the number of persons who, each year, reach the birthday that marks the beginning of the age interval indicated in column 1, and column 4 shows the number of persons who die each year in the indicated age interval.
Column 5 shows the number of persons in the stationary population in the indicated age interval. For example, the figure given for males in the age interval 20-25 years is 488,013. This means that in a stationary population of males supported by 100,000 annual births and with proportions dying in each age group always in accordance with column 2, a census taken on any date would show 488,013 persons between exact ages 20 and 25 years.
Column 6 shows the total number of persons in the stationary population (column 5) in the indicated age interval and all subsequent age intervals. For example, in the stationary population of males referred to in the last illustration, column 6 shows that there would be at any given moment a total of 5,240,576 persons who have passed their 20th birthday. The male population at all ages 0 and above (the total male population of the stationary community) would be 7,215,004.
Column 7-Average remaining lifetime ( o e x )-The average remaining lifetime (also called expectation of life) at any given age is the average number of years remaining to be lived by those surviving to that age on the basis of a given set of age-specific rates of dying. To arrive at this value, it is first necessary to observe that the figures in column 5 of the life table can also be interpreted in terms of a single life table cohort without introducing the concept of the stationary population. From this point of view, each figure in column 5 represents the total time (in years) lived between two indicated birthdays by all those reaching the earlier birthday among the survivors of a cohort of 100,000 live births. Thus the figure 488,013 for males in the age interval 20-25 is the total number of years lived between the 20th and 25th birthdays by the 97,997 (column 3) who reached the 20th birthday out of 100,000 males born alive. The corresponding figure 5,240,576 in column 6 is the total number of years lived after attaining age 20 by the 97,997 reaching that age. This number of years divided by the number of persons (5,240,576 divided by 97,997) gives 53.5 years as the average remaining lifetime of males at age 20.