Chapter 5: Social and Economic Environments
Direct Age-Adjustment
Because demographic factors can critically influence health outcomes, it is important to remove the
effect of such factors from population health statistics. If this is not done, mortality and morbidity
figures can be heavily influenced by the demographic composition of the population at the expense of
revealing important disease patterns. Adjustment can be performed on any population factor, such as
gender or ethnicity, but age is the most commonly adjusted factor. The procedure described here—the
direct method—is the most commonly used.
In direct age-adjustment, mortality figures are standardized to reflect the population structure of a
reference population. The same reference population must be used for all populations that are compared.
Although almost any population could be used as the reference population, many health organizations
use the same reference population so that all rates they report can be directly compared. Currently, the
US CDC uses 2000 census data to define its reference population (Hoyert and Anderson 2001); the
WHO has similarly published a standard world reference population that it recommends for comparing
rates across countries (Ahmad et al. 2001).
Direct age-adjustment takes the mortality rates for different age cohorts within each study population
and then recalculates them as if the study populations had the same population structure as the reference
population. It is therefore necessary to know: 1) the number of deaths in each age cohort of each study
population, 2) the population structure of each study population, and 3) the population structure of the
reference population.
Anthamatten and Hazen (2011), An Introduction to the Geography of Health
In the following example, we calculate the direct age-adjusted mortality rate for England and Wales in
2006 (the crude mortality rate for this population is 945 per 100,000). We first calculate age-specific
mortality rates by dividing the number of deaths in each age group in 2006 by the total population in
those age groups during the same year. The results are then usually expressed per 100,000 people. For
example, among 0-4 year olds, there were 3,930 deaths out of a total population of 2,488,142 in that age
group. We therefore divide the number of deaths (A) by the population (B) and multiply the result by
100,000, yielding an age-specific mortality rate of 157.9 per 100,000 (C).
0-4
5-14
15-24
25-34
35-44
45-54
55-64
65-74
75-84
85+
England and Wales, 2006
(A)
(E)
Number of
(B)
(C)
(D)
Weighted
Deaths
Population
Rate / 100,000
Reference Pop. Rate
3,930
2,488,142
157.9
0.0886
14.0
760
6,572,222
11.6
0.1729
2.0
2,870
7,051,638
40.7
0.1669
6.8
4,510
7,055,315
63.9
0.1554
9.9
10,120
8,207,854
123.3
0.1374
16.9
20,400
6,912,149
295.1
0.1141
33.7
45,500
6,332,792
718.5
0.0827
59.4
83,400
4,438,840
1878.9
0.0517
97.1
163,100
3,042,648
5360.5
0.0243
130.3
168,100
1,122,105
0.0064
95.1
14980.8
465.3
Once we have calculated age-specific mortality rates for all age groups, we must assign each rate a
weight according to the proportion of people in that age group in the reference population. In this
example, we use the reference population structure issued by the WHO. The numbers in column D
indicate the proportion of the world population that falls within each age cohort. Our task is to weight
each cohort from the England and Wales according to the reference population. For example, since the
5-14 years group is 0.1729 of the total reference population, the England and Wales age-specific
mortality rate for this age group should also constitute that fraction of the total age-adjusted figure for
the England and Wales population. To achieve this, we multiply each age-specific rate (C) by the
fraction of the reference population represented by that age group (D) to obtain the weighted rate (E).
Finally, we add all the weighted rates from column E to determine the age-adjusted rate (465 per
100,000). This age-adjusted rate can then fairly be compared with the age-adjusted rates of other
populations with different demographic structures, as long as they have been calculated with the same
reference population.
Anthamatten and Hazen (2011), An Introduction to the Geography of Health
Exercise
1. Using these techniques, calculate the direct age-adjusted mortality rate for the US in 2006 using
the same reference population as above. (The crude mortality rate for the US population is 810
per 100,000).
0-4
5-14
15-24
25-34
35-44
45-54
55-64
65-74
75-84
85+
United States, 2006
(A)
(C)
Number of
(B)
Rate /
Deaths
Population
100,000
33,158
20,436,496
6,149
40,453,947
34,887
42,441,606
42,952
40,406,397
83,043
43,660,883
185,031
43,282,105
281,401
31,586,149
390,039
18,914,650
667,338
13,046,868
701,992
5,296,814
(D)
(E)
Reference
Weighted
Pop.
Rate
0.0886
0.1729
0.1669
0.1554
0.1374
0.1141
0.0827
0.0517
0.0243
0.0064
2. Compare the resulting age-adjusted mortality rates for the US with those from England and
Wales that we calculated above. Which country has the lower age-adjusted mortality rate? What
does this mean? What hypotheses can you develop that might help explain this difference?
Sources
Ahmad, O. B., Boschi-Pinto, C., Lopez, A. D., Murray, C. J. L., Lozano, R. and Inoue, M. (2001) Age
Standardization of Rates: A new WHO Standard, Geneva: WHO.
Hoyert, D. L. and Anderson, R. N. (2001) "Age-adjusted Death Rates: Trend Data Based on the Year
2000 Standard Population", National Vital Statistics Reports, 49: 1-7.
Anthamatten and Hazen (2011), An Introduction to the Geography of Health