1957 - United Nations Statistics Division

in those conditions than do conventional central death
rates.
Three principal functions are given in this Demographic
Yearbook: (I) the probability of dying within
one year or within a specified age interval for persons
of exact ages specified (q a); (2) the survivors at the beginning
of each age interval out of a generation of 100,000
(1m); and (3) the mean expectation of life or the average
future lifetime for persons at exact ages specified (em).
Details of procedure in life table construction vary somewhat,
depending in large measure upon the availability
and form of basic data. The values of the life-table mortality
rate (q x), which is the basic function of the life
table, are generally so derived as to correspond to the
death rates of persons of that age in a particular period
of time. The basic data ordinarily consist of deaths in a
given period classified by age and sex and the mean
population of each age and sex during that period.
The most usual procedure is to utilize statistics of
deaths in a three-year or five-year period centred at the
census date so that the census results can be taken as the
mean population. Sometimes the period covered is that
between two censuses, and the population figures utilized
are averages of the results of the two censuses. It is common
to make adjustments to correct for misstatements of
age in the census and death statistics and in order to
obtain successive values of qm which vary smoothly from
one age to the next; a correction for the effect of migration
is occasionally made. In a few rare cases (for example,
in Brazil and India), approximate life tables have
been computed without the use of data on deaths, by
deriving qm values from the comparison of the population
in each age group as enumerated at one census with the
survivors at the next census.
Though first developed for actuarial purposes, the life
table has many applications in the demographic field.
Among these are: (I) the preparation of population projections
by age and sex; (2) analysis of effects of mortality
on the age and sex composition of a population; (3) comparisons
of summarizing measures of mortality, such as
the life-table death rate (the reciprocal of the expectation
of life at birth), expectation of life at various ages, etc.;
(4) computation of net reproduction rates; and (5) the
appraisal of the accuracy of census enumerations and
vital-registration data. In addition, life-table techniques
have been applied to the analysis of other types of demographic
data, for example, in the computation of probabilities
of marriage, specific for age and sex, on the basis
of census data classified by marital status.
The accuracy of life tables depends mainly upon the
accuracy and completeness of the registration of deaths
and of the enumeration of the population at the census.
Deficiencies in death registration are likely to be greater
than corresponding defects in census enumeration. Where
this happens, death rates are understated and the I" (survivors)
and ex values are exaggerated. The mortality rates
computed from population and death statistics at the very
young ages are particularly likely to be understated, and
such an error affects the Ix values throughout the table.
As indicated earlier, infant mortality rates obtained by
relating the number of infant deaths to the number of
births in countries where registration is deficient may be
either too low or too high, and the error in either direction
may have an important effect on the I" values for
41
every age. The expectation of life at birth (eo) will also
be too high or too low.
The accuracy and the international comparability of
life-table values are particularly suspect at the highest ages.
Those values depend on imperfect data and frequently
on somewhat arbitrary procedures. Certain remarkable
features-for example, the fact that the expectation of
life at ages over sixty is often distinctly higher in countries
where mortality in general is heavy than in countrie3
with low death rates-may, thus, merely reflect imperfections
of the tables.
Differences in the methods used for constructing life
tables (adjustment of data, graduation, etc.) may affect
the reliability of the results and impair their international
comparability. The effect of such differences is, however,
probably much smaller than that of deficiencies in censuses
and in death registration.
Table 24
Table 24 gives the "expectation of life" (em) or the
average number of years of life remaining for males and
females reaching the ages specified.
This table shows values from each life table-complete
or abridged-beginning about 1900. Male and female
expectations are shown separately for selected ages beginning
at birth (age 0) and proceeding with age 1, 2, 5, 10,
15, 20, 30, 40, 50, 60, 65, and 70 years. In some cases,
values were available only for the ages ending in 5. These
are given in the table preceded by a new run-in box-head.
Data are shown to the number of digits provided in the
official computation, thus producing a mixture of one
and two decimal values.
Coverage: Values are available in Table 24 for 76 geographic
areas. Lack of reliable base data restricts the
number of areas for which life tables are available but
even so, the coverage has increased notably in the last
few years. ''\There national data are not available, values
for sub-national areas-such as cities or provinces-are
shown. These sub-national values are presented in small
type to distinguish them from national data.
Limitations: Expectation-of-life values are subject to all the
limitations set forth above in the discussion of the accuracy
of the life table and, therefore, fundamentally to the
limitations of population and mortality statistics in
general. The values in Table 24 are to be interpreted
strictly in terms of the underlying assumption that surviving
cohorts are subjected to the age-specific mortality
rates of the period to which the life table refers. Thus,
in interpreting e 50 in the 1945-1950 life table for N OTWay
it may be said that, if males reaching age 50 were to experience
for the rest of their lives the same age-specific
mortality rates that obtained during the period 1945­
1950, they would, on the average, live 26.4 years past the
age of 50 or to 76.4 years of age.
Table 25
Life-table mortality rates 1900-1956 for males and
females, that is, the probability of dying within one year
or within a specified age interval for men and women
who have reached the exact age specified in the box-head,
are shown in Table 25. These are the basic functions of