demographic yearbook annuaire demographique 1951

correspondence is not perfect, of course, because the agedistributions
are not completely regular in shape.
The median age is that age which divides the population
in two equal parts, one half being older and one half being
younger than the median. The distribution of different
populations with the same median can differ greatly from
one another both above and below the median. However,
it is a convenient summarizing measure of the age composition
of a population and in general reflects the kinds of
differences with which we are here concerned. The percentage
70 years old and over shows an equally close correspondence
to the age-adjusted rates of Area A and underlines
the importance of the relative numbers of older persons
in determining the level of the crude death rate. Therefore,
in comparing the crude death rate of a young population
with the crude death rate of an older population it should
be kept in mind that, from the point of view of age-specific
mortality, the rate of the younger population is likely to be
understated relative to the rate of the older population.
The factor of age is also important in interpreting trends
in mortality as measured by changes over time in the crude
death rate. If a decline in the rate is accompanied by an
"aging" of the population (i.e., a rise in the median age as
increasing proportions of the population survive to the older
ages), the actual decrease in age-specific mortality is understated
by the change in the crude rate. By the same token,
if the crude rate remains stable or rises while the population
ages this does not necessarily mean that age-specific mortality
has stabilized or increased. In fact, it can happen that
age-specific mortality has actually decreased. For example,
the crude rate of Australia rose from 9.4 in 1936 to 9.7 in
1947. A comparison of age-specific death rates for these
two years shows that there was a decrease at every age
(see table 17). Thus the aging of the population in the space
of 11 years was sufficient to obscure the decline in mortality
as measured by the crude rate.
If, on the other hand, a decline in the crude death rate is
accompanied by an increase in the proportion of the population
at younger ages (or a fall in the median age), the
TABLE
Age-adjusted death rates of Area A and Area B based on the
age distributions of specified populations
B
Median Percentage
Age-adjusted death rates age 70years
oj base old and
Base population Area A Area B population over
(1) (2) (3) (4)
England & Wales (1948) . 13.2 28.7 34.8 6.6
Sweden (1948) ......... 12.8 28.5 33.9 6.2
Ireland (1946) ......... 12.5 28.5 29.1 6.8
New Zealand (1945) ..... 11.4 27.5 31.3 5.0
England & Wales (1931). 10.1 24.0 30.3 4.2
United States (1940) .... 9.7 23.6 29.0 4.0
Netherlands (1930) ...... 9.4 24.6 25.6 3.6
Bulgaria (1934) ......... 9.0 23.8 23.8 3.2
Japan (1947) ........... 8.2 24.8 22.3 2.6
Jamaica (1943) ......... 8.1 23.7 22.2 2.8
British Guiana (1946) .... 8.0 25.0 21.4 2.1
Israel (1948):
Jewish population..... 7.8 23.4 27.1 2.3
Honduras (1945) ....... 7.6 25.6 18.7 1.7
Venezuela (1941) ....... 6.9 23.8 19.3 1.6
Thailand (1947) ........ 6.7 23.8 18.4 1.4
Algeria (1948):
Moslem population.... 6.5 22.6 18.4 1.4
actual decline in mortality may be overstated by the change
in the crude rate. It often happens, in areas of relatively
high mortality, that a more rapid reduction is made in the
death rate of infants and young children than in death rates
at other ages. This can result in so increasing the number
of survivors at the young ages that the median age of the
population is lowered. In areas of this type, a decline in the
crude rate may be reflecting the conquest of infant mortality
while death rates at other ages remain relatively unchanged.
In the long run, however, the effect of reduction in mortality
is to raise the average age of the population. Only if the
number of births increases sufficiently to offset the aging
effects of mortality, will the average age of the population
remain stable or decrease.
Mention has been made of the effect on crude rates of
varying differences between age-specific rates for different
areas and times. The effect of this factor can be judged by
comparing the rank and level of age-adjusted rates for
Area B with those of the age-adjusted rates for Area A
which have just been described. Such rates are shown in
column 2 of table B.
It will be seen at once that the relative difference between
Area A and Area B is greater for each pair of age-adjusted
rates than for their own crude rates (12.0 and 23.1 respectively).
Thus, if Area A and Area B had the same age
composition, the contrast between them would be more
striking.
Two other changes in relationship are apparent. In the
first place, the relative range is considerably less among the
Area B rates than among the Area A rates, the highest rate
being only 27 per cent higher than the lowest. It becomes
clear, then, that with a schedule of age-specific rates like
those of Area B, the effect of differences in age-composition,
or of aging of the population, is less important than is the
case with age-specific rates like those of Area A. The chief
reason for this is the higher death rates of infants and young
children, which mean that aging of the population has a
smaller effect on the all-ages rate than is the case with the A
rates. The latter rates, being relatively low at all except the
advanced ages, are more sensitive to aging.
The second effect of the higher age-specific death rates is
to alter the rank of the age-adjusted rates. The population
of England and Wales 1948 still yields the highest rate and
the population of Algeria the lowest rate, but the order of
the intervening rates is somewhat changed. This alteration
is attributable also to the increased influence of mortality
at the very young ages upon the total rate. For example,
the rate based on the population of the United States 1940
dropped from fifth place with the age-specific rates ofArea A
to fourteenth place with the age-specific rates of Area B.
In the United States in 1940 the percentage of the population
that was under 5 years of age was very low as a result
of the low birth rates during the preceding decade. Consequently
the higher death rates for this age-group did not
affect the total rate so strongly as they did in the case of
Honduras which had a high percentage of young children
in 1945 and which moved up from thirteenth place with the
rates of Area A to fifth place with the rates of Area B.
However, the general order remains, the older populations
tending to yield higher rates and the younger, lower rates.
In other words, the two sets of rates are positively correlated.
Special attention should be given to the effect on crude
rates of populations such as those of Ireland 1946 and
Israel 1948. The former is a population that has experienced
fairly heavy out-migration with the result that the propor-
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