EPID 503 – Class 6
Standardization
Last Class: Total Mortality Rate
# people who became newly dead (in some time and place)
# people in population
Same as incidence but all people
at risk
*On the population-level, often
prevalent disease is assumed to be
so rare that total population ~
population at risk for incidence
estimation
*Learned that for cervical cancer,
this was not the case
Last Class: Total Mortality Rate
# people who became newly dead (in some time and place)
# people in population
Rate vs. proportion (ie. risk): On
the population-level, we cannot
tally person-time, so these “rates”
on the large scale are assumed to
occur in one year to make people*1
year or person years
Estimated with mid-point
population to get best estimate of
average population size during
time period
Today We Discuss Age Standardization
• What’s the issue?
What we really want to know is –
Is a person more likely to die if they were a
member of population A as compared to
population B?
Even with the same age-specific rates, a population
that is younger will appear to have lower overall
mortality rates.
Unadjusted May Be Good for Funeral
Director but Problematic for Public Health
To Compare Across Populations We Need
Comparable Groups
Method 1 asks “How would
the rates of death compare
in two populations if they
had the same age
distribution?”
What is this method called?
When is it most useful?
Direct Standardization Explained
Age-Specific Rates
from Population A
Age-Specific Rates
from Population B
Applied to
The age distribution of a
standard population
(eg. US population in 2000)
Note: Standard population is
somewhat arbitrary
Because using agespecific death rates
from populations
typically only used
in large groups
How to Implement the Direct Method
For each population:
(1) Calculate age-specific rates
(2) Multiply age-specific rates by the # of people in
corresponding age range in standard population (generates
the expected deaths in each age group of the standard population if
they had the rates of your population)
(3) Sum the expected # of deaths across all age groups
(generates the total expected deaths in the standard population if
they had the rates of your population)
(4) Divide total # of expected deaths by total standard
population (generates the mortality rate in standard population if
they had the rates of your population )
Result: Age-adjusted mortality rate for your population that
can now be compared to crude from standard or other
similarly standardized rates
To Compare Across Populations We Need
Comparable Groups
Method 2 asks “How many
deaths would I have expected
if this population had the same
mortality rates as some
standard population (e.g. the
US)?”
What is this method called?
When do we pick this method?
Indirect Method of Standardization
Rates from the
Standard population
Useful when I
don’t have or
trust the groupspecific rates
(i.e. population
is too small)
Applied to the age distribution of
the study population
How to Implement the Indirect Method
(1) Acquire age-specific mortality rates for standard
population
(2) Multiply standard population’s age-specific rates by # of
people in age range in study population (generates the
expected number of deaths in your population if it had the mortality
rate of the standard)
(3) Sum expected # of deaths across age groups in study
population (generates the total number of expected deaths in your
population if it had the mortality rates of the standard)
(4) Divide observed # of deaths by expected # of deaths in
study population (observed/expected)
Result: SMR (>1 more than expected, =1 as expected, <1 less
than expected)
Let’s Look at Populations with the Same
Age-Specific Rates but 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
With this Distribution, Let’s Estimate
the Crude Rates that Would be Observed
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
0.002*1700 + 0.005*0 + 0.01*0 =
17000
0.002* (1700/1700) + 0.005*(0/1700) + 0.01* (0/1700) =
0.002*1 + 0.005*0 + 0.01*0 = 0.002
We’re basically just shifting the overall rate to more closely
resemble the rates in the groups with the most number of people
Summary of Direct and Indirect
Adjustment
Direct
Population
Structure
Standard
Age-specific death
Rates
Observed
Indirect
Observed
Standard