Standardization of rates
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STANDARDIZATION OF RATES
Halyna Lugova, MD, PhD
October 8, 2014
Standardization of Rates: Town B: affluent rural
community, popular retirement area
The all-cause crude death rate – 14.2 per 1,000/year in 2013
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Town A: high unemployment rates, poverty
The all-cause crude death rate – 11.1 per 1,000/year in 2013
This suggests that mortality is higher in Town B
although this is not what we would expect given
socioeconomic characteristics of the two towns
Standardization of Rates: Age distribution for two populations
3
0
5
10
15
20
25
30
35
40
45
0-4 5-14 15-24 25-34 35-44 45-54 55-64 65-74 75-84 85 andover
Town A
Town B
X 1
,00
0 p
op
ula
tio
n
age
Standardization of Rates
4
Town A has a younger population, therefore it has a lower
death rate.
How to compare the two towns, independently of the
effects of this difference in age distribution?
We need to have a summary measure of mortality for all
age groups to avoid many tables of rates for each age
group.
Such summary measure that takes account of the
differences in age distribution of the two areas could be
derived by a technique called standardization.
Standardization of Rates 1. The indirect method provides standardized mortality
ratio (SMR) and indirectly standardized rates
2. The direct method provides directly standardized
rates
3. For indirect method we need to select standard
population, e.g. country population, or one of the
‘standard populations’ (not real) created to represent
population structure: World standard population,
European standard population, etc.
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Indirect Standardization – First, we will calculate the SMR for Town A
1. We need to know total number of deaths (‘observed’) in
Town A in 2013
2. We need to know the population in Town A in each age
group in 2013
Indirect Standardization
Age group Deaths
‘observed’: Town A
Population: Town A
Country death rate per 1000/year
(standard population)
Deaths ‘expected’: Town A
0-4 12400
5-14 26900
15-24 42000
25-34 32900
35-44 31700
45-54 27200
55-64 21600
65-74 18400
75-84 11300
85+ 3200
Total 2520 227600
Indirect Standardization 1. We need to know ‘observed’ deaths in Town A
2. We need to know the population in Town A in each age
group
3. We choose age-specific deaths rates for a ‘standard’
population; in this case hypothetical country
population
Indirect Standardization
Age group
Deaths ‘observed’:
Town A
Population: Town A
Country death rate per 1000/year (standard
population)
Deaths ‘expected’: Town
A
0-4 12400 1.50
5-14 26900 0.03
15-24 42000 0.32
25-34 32900 0.64
35-44 31700 2.34
45-54 27200 4.02
55-64 21600 6.69
65-74 18400 14.32
75-84 11300 78.30
85+ 3200 180.20
Total 2520 227600
Indirect Standardization 1. We need to know ‘observed’ deaths in Town A
2. We need to know the population in Town A in each age
group
3. We choose age-specific deaths rates for a ‘standard’
population
4. We calculate the numbers of deaths that would have
occurred in Town A – expected deaths, in each age
group, if the ‘standard’ population death rates had
applied.
Indirect Standardization
• For that, we need to multiply the country rate
(column 4) by the Town A population (column 3) in
the same age group
For example, for the 0-4 age group:
𝟏.𝟓𝒙𝟏𝟐𝟒𝟎𝟎
𝟏𝟎𝟎𝟎 = 18.6
• Finally, add up all the age-specific expected deaths to
obtain total number of expected deaths
Indirect Standardization
Age group Deaths
‘observed’: Town A
Population: Town A
Country death rate per 1000/year
(standard population)
Deaths ‘expected’: Town
A
0-4 12400 1.50 18.60
5-14 26900 0.03 0.81
15-24 42000 0.32 13.44
25-34 32900 0.64 21.06
35-44 31700 2.34 74.18
45-54 27200 4.02 109.34
55-64 21600 6.69 144.50
65-74 18400 14.32 263.49
75-84 11300 78.30 884.79
85+ 3200 180.20 576.64
Total 2520 227600 2106.85
Indirect Standardization
1. We need to know ‘observed’ deaths in Town A
2. We need to know the population in Town A in each age
group
3. We choose age-specific deaths rates for a ‘standard’
population
4. We calculate the numbers of expected deaths in town A in
each age group.
5. We can calculate the SMR now
Indirect Standardization
An SMR 120 means that, independently of the influence
of the age distribution in Town A, the overall mortality in
Town A is 20 per cent higher than country average (our
‘standard’ population).
SMR = 𝑻𝒐𝒕𝒂𝒍 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒐𝒃𝒔𝒆𝒓𝒗𝒆𝒅 𝒅𝒆𝒂𝒕𝒉𝒔
𝑻𝒐𝒕𝒂𝒍 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒆𝒙𝒑𝒆𝒄𝒕𝒆𝒅 𝒅𝒆𝒂𝒕𝒉𝒔 𝒙 𝟏𝟎𝟎
SMR = 𝟐𝟓𝟐𝟎
𝟐𝟏𝟎𝟔.𝟖𝟓 𝒙 𝟏𝟎𝟎 = 𝟏𝟏𝟗. 𝟔 ~ 𝟏𝟐𝟎
Indirect Standardization
– Interpretation of SMR
Independently of the influence of the age distribution, an
SMR
1. of 100% means no difference between the overall
mortality in the population of interest and in the
standard population.
2. >100% means that the overall mortality in the population
of interest is higher than in the standard population.
3. < 100% means that the overall mortality in the
population of interest is lower than in the standard
population.
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Indirect Standardization • To calculate SMR we need to know:
• Age-specific index population data
• Total number of deaths in the index population
• Age-specific deaths rates of the standard
population
• SMR can be calculated if the numbers of
deaths in each age group are not available
16
Indirect Standardization Now, we will calculate the SMR for Town B
Age group Deaths
‘observed’: Town B
Population: Town B
Country death rate per 1000/year (standard
population)
Deaths ‘expected’:
Town B
0-4 5200 1.50 7.80
5-14 15100 0.03 0.45
15-24 11300 0.32 3.62
25-34 11100 0.64 7.10
35-44 16600 2.34 38.84
45-54 15400 4.02 61.91
55-64 14900 6.69 99.68
65-74 12700 14.32 181.86
75-84 8800 78.30 689.04
85+ 3100 180.20 558.62
Total 1626 114200 1648.93
Indirect Standardization –We will calculate the SMR for Town B
An SMR 99 means that, independently of the influence of
the age distribution in Town B, the overall mortality in
Town B does not differ significantly (very close to 100)
from that for the ‘standard’ population.
SMR = 𝑻𝒐𝒕𝒂𝒍 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒐𝒃𝒔𝒆𝒓𝒗𝒆𝒅 𝒅𝒆𝒂𝒕𝒉𝒔
𝑻𝒐𝒕𝒂𝒍 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒆𝒙𝒑𝒆𝒄𝒕𝒆𝒅 𝒅𝒆𝒂𝒕𝒉𝒔 𝒙 𝟏𝟎𝟎
SMR = 𝟏𝟔𝟐𝟔
𝟏𝟔𝟒𝟖.𝟗𝟑 𝒙 𝟏𝟎𝟎 = 𝟗𝟖. 𝟔 ~ 𝟗𝟗
Indirect Standardization –Comparison of SMRs
We cannot compare SMRs between two populations,
e.g. Town A and Town B - only to the standard
population – because the age-specific rate have been
applied to two different populations
What we can state, is that mortality in Town A is 20 per
cent higher than the country average. For Town B,
mortality does not differ significantly from the country
level (very close to 100).
19
Direct Standardization
–Direct standardization allows direct comparison
between two populations
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Direct Standardization – We will use population of Town A as the standard and
standardize population of Town B against it
– We will first look at crude death rate for people aged 65 and
over
Town A (standard)
No. of population
Deaths observed
Age specific rate/1000 per year
65-74 18400
75-84 11300
85+ 320
30020 297 9.89
Town B
No. of population
Deaths observed
Age specific rate/1000 per year
65-74 12700 45
75-84 880 93
85+ 310 220
13890 358 25.77
Direct Standardization – Now, we will calculate age-specific rates for Town B
Town A (standard)
No. of population
Deaths observed
Age specific rate/1000 per year
65-74 18400
75-84 11300
85+ 320
30020 297 9.89
Town B
No. of population
Deaths observed
Age specific rate/1000 per year
65-74 12700 45 3.54
75-84 880 93 105.68
85+ 310 220 709.68
13890 358 25.77
Direct Standardization – Standardize the Town B rate to the Town A:
1. Apply age-specific rates of Town B to the age-specific
groups of Town A to calculate the expected numbers in
each group
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Age-specific rate for Town B
Town A (standard) population
Expected deaths for Town B
65-74 3.54 18400 65.20
75-84 105.68 11300 1194.20
85+ 709.68 320 227.10
30020
Direct Standardization
– Standardize the Town B rate to the Town A:
2. Add up the expected deaths to obtain the total
3. Divide the total expected cases by the total Town A (standard ) population to obtain the age-standardized death rate (x 1,000)
24
Age-adjusted rate for Town B
Town A (standard) population
Expected deaths for Town B
65-74 3.54 18400 65.20
75-84 105.68 11300 1194.20
85+ 709.68 320 227.10
49.52 30020 1486.50
Direct Standardization
– Interpretation
• Before standardization:
– Crude death rate in Town A for people aged 65 and over was 9.89 per 1000 per year
– Crude death rate in Town B for people aged 65 and over was 25.87 per 1000 per year (2.5 times higher than in Town A)
• After standardization of the population of Town B crude death rate to the population of Town A:
– Age-adjusted death rate in Town B for people aged 65 and over was 49.52 per 1000 per year (5 times higher than in Town A)
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Direct Standardization
– To calculate age-adjusted deaths rates we need
to know:
• Age-specific index population data
• Number of deaths in each age group in index
population
• Age-specific standard population data
26
SUMMARY Standardization is applicable for factors other
than age (socio-economic status, race, area of
residence)
Any rates can be standardized, e.g. incidence
Standardization is required to adjust rates for
influence of factors, e.g. age, which could have
impact on the comparison of those rates
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SUMMARY
There are two methods: indirect and direct standardization
Indirect standardization applies age-specific rates from the standard population to the numbers of people in each age group in the index population
Direct standardization applies age-specific rates from the index population to the numbers of people in each age group of a standard population
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SUMMARY
Indirect standardization (calculation of SMR) does not require age-specific rates in the index population
Indirect standardization does not allow direct comparison between SMRs
Indirect standardization is more precise
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