EPIDEMIOLOGY 373
WORLD STANDARDIZED RATESAnother method of standardization, essentially similar to that
described above, makes use of standard population, defi ned
in terms of the numbers in each age group. The rates of each
population are applied to this standard population to obtain
a set of expected mortality deaths and thus a rate standard-
ized to the standard population. It is becoming increasingly
common today to use a constructed “world standard popula-
tion” for this purpose, so that rates so obtained are described
as “world standardized rates” (WSRs). This concept was cre-
ated originally by the late Professor Mitsui Sigi, a Japanese
epidemiologist, when attempting to compare cancer mortal-
ity rates between different countries throughout the world.
The age structure of a developing country (often typifi ed as
Africa) has a triangular form when depicted as a pyramid, at
least before the onset of AIDS (see Figure 3), with a small
proportion of the elderly, but its proportion increasing regu-
larly toward the lowest age groups. A typical pyramid for a
developed country (typifi ed as European) is that in Figure 4,
which shows a rather more stable pattern until the ultimate
triangle at the upper end.
These forms of standardization have been disrupted by
HIV, which is the causative agent of AIDS. In Botswana for
the year 2020 it has been predicted that there will be a larger
population around the age group 60–70s than for 40–50s as a
result of AIDS (Figure 5). The dramatic effect on the popula-
tion structure of this virus will change how age adjustment
must be performed for many of the affected countries. Thus,
in the future, age adjustment will not be as straightforward
as described in many standard epidemiology textbooks.INDIRECT STANDARDIZATIONWhen the objective is to compare the mortality rates of var-
ious subpopulations, such as geographical, occupational, orother subdivisions of a single country, a different method
is commonly used. What has already been described is
known as the “direct method” of standardization, using a
standard population to which the rates for various coun-
tries are applied. The “indirect method” of standardization
makes use of a standardized set of mortality rates by age
group, and these rates are applied, age by age, to each of
the subpopulations, providing thereby a total of expected
deaths; the actual total of deaths observed in each subpopu-
lation is then divided by the expected total to provide what
is known as the “standardized mortality ratio” (SMR). The
standard set of mortality rates used is that of the overall
population’s experience, and almost invariably that popula-
tion is the sum of all the subpopulations. Clearly if some
SMRs are greater than 100 (it is conventional to multiply
the SMR by 100, which has the convenience of making
apparent the percentage difference from expectation), then
some will be below, since the weighted mean of the SMRs
must be 100.
For the purposes of comparisons of this type, the indirect
method has a number of advantages over the direct method.
Several of the subpopulations may be quite small in size,
especially in some age groups where the numbers observed
may be very small, so that age-specifi c mortality rates can
fl uctuate widely. The mortality rates of the parent popula-
tion, on the other hand, are inherently more stable than those
of any fractional subpopulation. The structure by age of each
subpopulation will in general be easily obtainable, often
from the census, with reasonable accuracy, and so will the
total number of deaths. The ratio of observed to expected
deaths—the SMR—is then easily interpreted as a percentage
above or below expectation. An assessment of the statisti-
cal signifi cance of its difference from 100 can be obtained
by assuming a distribution similar to the Poisson, so that
the standard error would be 100 E , where E is the expected
number of deaths: deviations from 100 of more than twice
this quantity would be regarded as statistically signifi cant at20 15 10 5 0 0 5 10 15 20051015202530354045505560657075808590Males (%) Females (%)FIGURE 3 Population pyramid: a developing country.20 15 10 5 0 0 5 10 15 20051015202530354045505560657075808590Males (%) Females (%)
FIGURE 4 Population pyramid: a developed country.C005_011_r03.indd 373C005_011_r03.indd 373 11/18/2005 10:25:40 AM11/18/2005 10:25:40 AM