Standardization of Rates

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

Crude rates Are summary statistics that ignore the heterogeneity of the population under investigation

Crude Mortality Rates Number of deaths in a specified year ___________________  1000 Number of individuals in the population in the specified year

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)

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

Age-Specific Mortality Rate Number of deaths among persons aged 0-14 in a given year Aged 0 –14 = _________________________  years Total number of persons aged 0-14 in the same year

Standardization of Rates Used to reduce distortion in comparisons between crude areas Also referred to as adjusting rates

Adjusting Rates Allows comparisons of rates between populations that differ by variables that can influence the rate (e.g., age) Direct method Indirect method

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

Specific Rates Advantages Homogenous subgroups Detailed rates useful for public health and Epidemiological aims Disadvantages Cumbersome to compare subgroups of two or more populations

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

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

Direct 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) PopulationDeathsDeath Rate (per 1000) PopulationDeathsDeath Rate (per 1000) Under 11, , – 143, , – 346, , – 5413, , – 647, , Over 6420,0001, ,0001, All ages50,0001, ,0001, Population, Deaths, and Death Rate by Community and by Age Community 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 16, – 1423, – 3441, – 5430, – 6415, Over 6435, , ,150 Total150,00035,63, ,772.5 Age – adjusted death rate (per 1000)

Indirect Adjustment of Rates Used if age-specific rates cannot be estimated Mirror image of the direct method

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

Total observed deaths in a population ____________________ Total expected deaths in a population

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, – 143, – 346, – 5413, – 647, Over 6420, Total50,

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, – 143, – 346, – 5413, – 647, Over 6420, ,800.0 Total50, ,032.5 SMR A = 1781 / = SMR B = 1.0

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