Standardization of Rates. Rates of Disease Are the basic measure of disease occurrence because they most clearly express probability or risk of disease.

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