1
CLASS SESSION 6
(AGE) STANDARDIZATION
Epidemiology 503, Section 2
Last Class


We looked at death certificates and mortality rates
for stroke
Total (crude) mortality rate
# people who became newly dead (in some time and place)
# people in population

Case fatality “rate”
# deaths due to a specific disease
Total # people with that disease
Proportional Mortality
3
Importance of:
Place
Time
Population
4
Age is major contributor to mortality
5
Specific
Thought question
6
If we want to know:
Is a person more likely to die if they
were a member of population A as
compared to population B?
What’s the issue?
7

Even with the same age-specific rates, a population
that is younger will appear to have lower overall
mortality rates.
Why are unadjusted mortality rates
problematic?
8
To Compare Across
Populations We
Need Comparable
Groups
Today: Age Standardization
Sweden
9 per 1000
10
Old
Middle
aged
Young

5
3
Crude
Mortality
Rate
Age-Specific
Mortality
Rates
Panama
7 per 1000
15
10
5
Old
Middle
aged
Young
Adjusted rates allow comparisons of mortality in different
populations after removing the confounding effects of age
Two Methods

Direct Method
Used if age-specific death rates in a population of interest
are known
 “How would the rates of death compare in 2 populations if
they had the same age distribution?”


Indirect Method
Used if age-specific death rates of the population for
standardization are unknown or unstable, for example,
because the population is small.
 “How many deaths would I have expected if this population
had the same mortality rates as some standard population?”

Because using age-specific death rates
from populations typically only used in
large groups
Direct Method
Rates from
Population A
Rates from
Population B
Applied to
For each population:
(1)
(2)
(3)
A standard population e.g. US
population in 2000
Choice of standard is
somewhat ARBITRARY.
(4)
Calculate age-specific mortality
rates
Multiply age-specific rates by the #
of people in corresponding age
range in standard population
Sum expected # of deaths across
age groups
Divide total # of expected deaths
by total standard population
Result: Age-adjusted mortality rate for
each population of interest
Useful when I don’t have or trust the groupspecific rates (i.e. population is too small)
Indirect Method
(1)
Rates from the
standard population
Applied to the age
distribution of the
study population
(2)
(3)
(4)
Acquire age-specific mortality rates for
standard population
Multiply standard population’s agespecific rates by # of people in age
range in population of interest
Sum expected # of deaths across age
groups in study population
Divide observed # of deaths by
expected # of deaths in population of
interest
SMR: observed # deaths per year
expected # deaths per year
>1 more deaths than expected
=1 as expected
<1 less deaths than expected
Example: Same Age-Specific Rates to
Populations with Different Ages
Rate
N
Young
0.002
1000
Middle
0.005
500
Old
0.010
200
Total
1700
Rate
N
Young
0.002
200
Middle
0.005
500
Old
0.010
1000
Total
1700
What are the crude rates?
Rate
N
Expected Deaths
Young
0.002
1000
1000*0.002 = 2
Middle
0.005
500
500*0.005 = 2.5
Old
0.010
200
200*0.01 = 2
1700
6.5
6.5/1700=0.0038
Rate
N
Expected Deaths
Rate
Young
0.002
200
200*0.002 = 0.4
Middle
0.005
500
500*0.005 = 2.5
Old
0.010
1000
1000*0.01 = 10
1700
12.9
Total
Total
Rate
12.9/1700=0.0076
Mathematically It’s a Weighted Average
Rate
N
Young
0.002
1700
Middle
0.005
0
Old
0.010
0
1700*0.002 + 0*0.005 + 0*0.01 =
17000
We’re basically just shifting the overall rate to more closely
resemble the rates in the groups with the most number of people
Decision: Direct or Indirect?
Do you have or are you able to
calculate age-specific rates for your
population(s) of interest?
Yes  Direct method
No  Indirect method
Comparing Standardized Mortality Rates
Direct standardization: Expected rate (or standardized
rate) can be compared to the crude rate or to any
other similarly standardized rate.
Indirect standardization: Expected number of deaths
can be compared to the number of actual deaths with
the SMR.