Standardization of Rates

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Standardization of Rates
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Rates of Disease

• Are the basic measure of disease occurrence
• because they most clearly express probability
• or risk of disease in a defined population over
• a period of time
• Incidence
• Prevalence
• Mortality

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Crude rates
Are summary statistics that ignore the
heterogeneity of the population under
investigation
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Crude Mortality Rates
Number of deaths in a specified
year ___________________ ? 1000 Number of
individuals in the population in the specified
year
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Specific Rates
Stratifies populations into more
homogeneous groups (strata) based on the
demographic characteristic thought to be related
to the outcome of interest (e.g. age-specific,
sex-specific, race-specific)
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Age-Specific Mortality Rate

• Provide a broader view of mortality for
• sub-groups stratified by age
• Numerator and denominator are limited
• to a specific age group
• Comparable across populations

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Age-Specific Mortality Rate
Number of deaths among persons aged
0-14 in a given year Aged 0 14
_________________________ ? 100000
years Total number of persons aged 0-14
in the same year
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Standardization of Rates
Used to reduce distortion in comparisons between
crude areas Also referred to as adjusting rates
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Adjusting Rates
Allows comparisons of rates between populations
that differ by variables that can influence the
rate (e.g., age) Direct method Indirect method
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Crude Rates
Advantages Actual Summary rates Easy calculation
for international comparisons Disadvantages Since
population vary in composition (e.g.,
age) differences in crude rates difficult to
interpret
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Specific Rates
Advantages Homogenous subgroups Detailed rates
useful for public health and Epidemiological
aims Disadvantages Cumbersome to compare
subgroups of two or more populations
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Adjusted Rates
Advantages Summary statement Differences in
group composition removed allows unbiased
comparison Disadvantages Fictional
rates Absolute magnitude dependent on standard
population chosen Opposing trends in subgroup
masked
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Direct Adjustments of Rates
Requires a standard population, to which the
estimated age-specific rates can be
applied Choice of the standard population may
affect the magnitude of the age-adjusted rates,
but not the ranking of the population
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Direct Adjustments of Rates
Multiply standard population by age-specific
rates for populations A and B to determine the
standardized rates Compare standardized rates
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Population, Deaths, and Death Rate by
Community and by Age
Community A Community A Community A Community A Community B Community B Community B
Age (year) Population Deaths Death Rate (per 1000) Population Deaths Death Rate (per 1000)
Under 1 1,000 15 15.0 5,000 100 20.0
1 14 3,000 3 1.0 20,000 35 1.0
15 34 6,000 6 1.0 35,000 35 1.0
35 54 13,000 52 4.0 17,000 85 5.0
55 64 7,000 105 15.0 8,000 160 20.0
Over 64 20,000 1,600 80.0 15,000 1,350 90.0
All ages 50,000 1,781 35.6 100,000 1,740 17.4
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Standard Population by Age and Age-Specific Death
Rates
Age (years) Standard population Death rate in A (per 1,000) Expected deaths at As rate Death rate in B (per 1,000) Expected deaths at Bs rate
Under 1 6,000 15.0 90 20.0 120.0
1 14 23,000 1.0 23 0.5 11.5
15 34 41,000 1.0 41 1.0 41.0
35 54 30,000 4.0 120 5.0 150.0
55 64 15,000 15.0 225 20.0 300.0
Over 64 35,000 80.0 2,800 90.0 3,150
Total 150,000 35,6 3,299 17.4 3,772.5
Age adjusted death rate (per 1000) 22.0 25.0
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Indirect Adjustment of Rates
Used if age-specific rates cannot be
estimated Mirror image of the direct method
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Indirect Adjustment of Rates
Based on applying the age-specific rates of the
standard population to the population of
interest to determine the number of expected
deaths ?
Standardized Mortality Ratio
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Standardized Mortality Ratio
Total observed deaths in a population ____________
________ Total expected deaths in a population
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Population of Community A by Age and Standard
Death Rates
Age (years) Population in A Standard death rate (per 1,000)
Under 1 1,000 20.0
1 14 3,000 0.5
15 34 6,000 1.0
35 54 13,000 5.0
55 64 7,000 20.0
Over 64 20,000 90.0
Total 50,000 17.4
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Population and Expected Deaths of Community A by
Age
Age (years) Population in A Standard death rate (per 1,000) Expected deaths in A at standard rates
Under 1 1,000 20.0 20.0
1 14 3,000 0.5 1.5
15 34 6,000 1.0 6.0
35 54 13,000 5.0 65.0
55 64 7,000 20.0 140.0
Over 64 20,000 90.0 1,800.0
Total 50,000 17.4 2,032.5
SMRA 1781 / 2032.5 0.876 SMRB 1.0
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Standardized Mortality Ratio
If the SMR is greater than 1, more deaths have
occurred than anticipated If the SMR is less
than 1, fewer deaths have occurred than
anticipated