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Published byBasil Shaw Modified over 9 years ago
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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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Community A Community B Age (year) PopulationDeathsDeath Rate (per 1000) PopulationDeathsDeath Rate (per 1000) Under 11,0001515.05,00010020.0 1 – 143,00031.020,000351.0 15 – 346,00061.035,000351.0 35 – 5413,000524.017,000855.0 55 – 647,00010515.08,00016020.0 Over 6420,0001,60080.015,0001,35090.0 All ages50,0001,78135.6100,0001,74017.4 Population, Deaths, and Death Rate by Community and by Age Community and by Age
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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 A’s rate Death rate in B (per 1,000) Expected deaths at B’s rate Under 16,00015.09020.0120.0 1 – 1423,0001.0230.511.5 15 – 3441,0001.0411.041.0 35 – 5430,0004.01205.0150.0 55 – 6415,00015.022520.0300.0 Over 6435,00080.02,80090.03,150 Total150,00035,63,29917.43,772.5 Age – adjusted death rate (per 1000) 22.025.0
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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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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 and Standard Death Rates Age (years) Population in A Standard death rate (per 1,000) Under 11,00020.0 1 – 143,0000.5 15 – 346,0001.0 35 – 5413,0005.0 55 – 647,00020.0 Over 6420,00090.0 Total50,00017.4
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Population and Expected Deaths of Community A by Age Age (years) Populat ion in A Standard death rate (per 1,000) Expected deaths in A at standard rates Under 11,00020.0 1 – 143,0000.51.5 15 – 346,0001.06.0 35 – 5413,0005.065.0 55 – 647,00020.0140.0 Over 6420,00090.01,800.0 Total50,00017.42,032.5 SMR A = 1781 / 2032.5 = 0.876 SMR B = 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
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