demographic yearbook annuaire demographique 1951

CHAPTER I.
RECENT MORTALITY TRENDS
The data brought together in the present volume indicate
a marked and widespread decline in mortality during the
past 20 years. In spite of the crudeness of some of the measures,
the deficiencies of some of the basic data and the fact
that certain geographic segments are not represented in the
statistics, the unanimity of available information, after allowance
for known defects, is rather overwhelming.
CRUDE DEATH RATES
The only direct measure of mortality that is available for
a considerable number of countries and over a fairly long
period of time is the crude death rate, in which the total
number of deaths is related to the total population (see rates
for the period 1930-1950 shown in table 14). If the basic
data are complete, this rate is a perfectly accurate measure
of the frequency with which deaths occur in a population
a\ld is useful for many purposes that do not require a deeper
analysis, for example, measuring overall rates of population
growth or estimating the total population. However, it is
subject to rather severe limitations for purposes of assessing
mortality trends and comparing the mortality levels ofdifferent
populations, for it is strongly affected by differences in
the age composition of the population as well as by differences
in the levels of mortality.
Age composition and crude rates. The crude death rate may
be regarded as an average in which the death rate for each
age-group is weighted by the number of that age in the total
population. From this point of view, it is at once apparent
that the crude rate is affected not only by variations in the
proportion of the population at each age, but also by variations
in the age-specific rates themselves. The pattern of agespecific
rates, while always forming aU-shaped curve, does
not change uniformly in time nor vary uniformly between
populations. Thus the relative differences between rates for
identical age-groups in two different populations vary widely
from age-group to age-group.
The effect of these two factors of variation in age composition
and variation in age-specific mortality can be illustrated
by reference to observed data on age and mortality. For this
purpose, two sets of data were selected, one representing
relatively low mortality (designated as "Area A") and the
other representing relatively high mortality (designated as
"Area B"). The crude death rate of Area A is 12.0; that of
Area B is 23.1. Both the' age composition and the agespecific
death rates of these two populations differ strongly
(see table A). The age-specific death rates of Area Bare
consistently higher than those of Area A, but the relative
difference ranges from a ratio of 14 to 1 for the age-group
1-4 to near equality for the group 85 and over. Consequently,
the difference between the rates for all ages (the
crude rates) cannot be taken as a measure of the difference
at each age. This difference cannot even be taken as representing
a comparable average of age-specific differences because
of the factor of age composition. The B population,
being younger, gives heavier weights to the death rates at
younger ages; the A population, on the other hand, gives
heavier weights to the rates at older ages.
The specific effect of differences in age composition can
be judged by applying the age-specific rates of a given country
at a given time to the population of other countries or
other times and comparing the resulting rates for all ages.
These "age-adjusted" rates are the crude rates that the given
country would have if its population were distributed by
age in the same way as that of the other countries.
Column 1 of table B gives age-adjusted rates for Area A,
computed from the age-specific death rates of Area A and
the population by age of 16 other areas for various dates
between 1930 and 1948. These populations were so selected
as to represent a broad range of population types. The data
show that, with the age distribution of Algeria in 1948,
Area A would have a crude death rate of only 6.5 per 1,000,
as compared with its observed rate of 12.0. On the other
hand, with the age distribution of England and Wales in
1948, Area A would have a rate of 13.2, more than twice
the rate yielded by the Algerian age distribution.
The age-adjusted rates are ranked in descending order in
the table and show that the older populations tend to yield
higher death rates than the younger. The median age and
the percentage 70 years old and over (given for the various
base populations in the last two columns of table B) show
a close correlation with the age-adjusted death rates. The
TABLE A
Age-specific death rates and percentage distribution of total
population by age: Selected areas
Death rates
Percentage
distribution
of population
Age Area A Area B Area A Area B
All ages........ 12.0 23.1 100.0 100.0
Under 1........ 45.9 205.3 1.3 2.7
1-4 .......... 3.5 48.2 5.2 11.8
5-9 .......... 1.5 7.9 7.1 14.4
10-14.......... 1.4 4.0 8.2 12.2
15-19.......... 2.6 6.0 8.8 10.2
20-24.......... 3.3 9.0 8.7 7.9
25-29.......... 3.0 10.0 8.8 8.1
30-34.......... 3.4 11.5 8.1 6.7
35-39.......... 3.8 13.2 7.4 7.0
40-44.......... 4.8 15.4 6.7 4.8
45-49.......... 6.5 18.0 6.1 3.9
50-54...... - .. 9.4 21.6 5.5 3.1
55-59 .......... 13.5 27.1 4.8 2.2
60-64.......... 20.2 40.4 4.0 2.1
65-69.......... 32.9 54.8 3.1 1.2
70-74.......... 53.5 84.7 2.8 0.8
75-79 .......... 90.5 108.0 1.9 0.4
80-84.......... 147.4 161.1 1.0 0.3
85+ ........... 260.9 295.2 0.5 0.2
9