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Standardization of RatesStandardization of Rates
Rates of DiseaseRates 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
Crude ratesCrude rates
Are summary statistics that ignore
the heterogeneity of the population
under investigation
Crude Mortality RatesCrude Mortality Rates
Number of deathsin a specified year___________________ 1000
Number of individualsin the populationin the specified year
Specific RatesSpecific 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)
Age-Specific Mortality RateAge-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
Age-Specific Mortality RateAge-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
Standardization of RatesStandardization of Rates
Used to reduce distortion in comparisons between crude areas
Also referred to as adjusting rates
Adjusting RatesAdjusting Rates
Allows comparisons of rates between
populations that differ by variables that
can influence the rate (e.g., age)
Direct method
Indirect method
Crude RatesCrude 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
Specific RatesSpecific Rates
Advantages
Homogenous subgroups
Detailed rates useful for public health and
Epidemiological aims
Disadvantages
Cumbersome to compare subgroups of two or
more populations
Adjusted RatesAdjusted 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
Direct Adjustments of RatesDirect 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
Direct Adjustments of RatesDirect Adjustments of Rates
Multiply standard population by age-specific rates
for populations A and B to determine the
standardized rates
Compare standardized rates
Community A 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
Population, Deaths, and Death Rate byPopulation, Deaths, and Death Rate by Community and by AgeCommunity and by Age
Standard Population by Age and Age-Specific Death Rates
Age
(years)
Standard population
Death rate
in A
(per 1,000)
Expected deaths at
A’s rate
Death rate
in B
(per 1,000)
Expected deaths at
B’s 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
Indirect Adjustment of RatesIndirect Adjustment of Rates
Used if age-specific rates cannot be estimated
Mirror image of the direct method
Indirect Adjustment of RatesIndirect 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
Standardized Mortality RatioStandardized Mortality Ratio
Total observed deathsin a population____________________
Total expected deathsin a population
Population of Community A by AgePopulation of Community A by Age and Standard Death Ratesand 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
Population and Expected Deaths of Population and Expected Deaths of Community A by AgeCommunity 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 = 0.876SMRB = 1.0
Standardized Mortality RatioStandardized 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