Class session 6 (AGE) Standardization Epidemiology 503, Section 2

Last Class We looked at death certificates and mortality rates for stroke Total (crude) mortality rate # people who became newly dead (in some time and place) # people in population Case fatality “rate” # deaths due to a specific disease Total # people with that disease

Proportional Mortality Importance of: Place Time Population

Age is major contributor to mortality Specific

Thought question If we want to know: Is a person more likely to die if they were a member of population A as compared to population B? What’s the issue?

Even with the same age-specific rates, a population that is younger will appear to have lower overall mortality rates.

Why are unadjusted mortality rates problematic? To Compare Across Populations We Need Comparable Groups

Today: Age Standardization Sweden Panama Crude Mortality Rate 9 per 1000 7 per 1000 10 5 3 15 10 5 Age-Specific Mortality Rates Old Old Middle aged Middle aged Young Young Adjusted rates allow comparisons of mortality in different populations after removing the confounding effects of age

Two Methods Direct Method Indirect Method Used if age-specific death rates in a population of interest are known “How would the rates of death compare in 2 populations if they had the same age distribution?” Indirect Method Used if age-specific death rates of the population for standardization are unknown or unstable, for example, because the population is small. “How many deaths would I have expected if this population had the same mortality rates as some standard population?”

Rates from Population A Rates from Population B Because using age-specific death rates from populations typically only used in large groups For each population: Calculate age-specific mortality rates Multiply age-specific rates by the # of people in corresponding age range in standard population Sum expected # of deaths across age groups Divide total # of expected deaths by total standard population Result: Age-adjusted mortality rate for each population of interest Direct Method Rates from Population A Rates from Population B Applied to A standard population e.g. US population in 2000 \ Choice of standard is somewhat ARBITRARY.

Applied to the age distribution of the study population Useful when I don’t have or trust the group- specific rates (i.e. population is too small) Acquire age-specific mortality rates for standard population Multiply standard population’s age- specific rates by # of people in age range in population of interest Sum expected # of deaths across age groups in study population Divide observed # of deaths by expected # of deaths in population of interest SMR: observed # deaths per year expected # deaths per year >1 more deaths than expected =1 as expected <1 less deaths than expected Indirect Method Rates from the standard population Applied to the age distribution of the study population

Example: Same Age-Specific Rates to Populations with Different Ages Young 0.002 1000 Middle 0.005 500 Old 0.010 200 Total 1700 Rate N Young 0.002 200 Middle 0.005 500 Old 0.010 1000 Total 1700

What are the crude rates? N Expected Deaths Young 0.002 1000 1000*0.002 = 2 Middle 0.005 500 500*0.005 = 2.5 Old 0.010 200 200*0.01 = 2 Total 1700 6.5 6.5/1700=0.0038 Rate N Expected Deaths Young 0.002 200 200*0.002 = 0.4 Middle 0.005 500 500*0.005 = 2.5 Old 0.010 1000 1000*0.01 = 10 Total 1700 12.9 12.9/1700=0.0076

Mathematically It’s a Weighted Average 1700*0.002 + 0*0.005 + 0*0.01 = 17000 We’re basically just shifting the overall rate to more closely resemble the rates in the groups with the most number of people Rate N Young 0.002 1700 Middle 0.005 Old 0.010

Decision: Direct or Indirect? Do you have or are you able to calculate age-specific rates for your population(s) of interest? Yes  Direct method No  Indirect method

Comparing Standardized Mortality Rates Direct standardization: Expected rate (or standardized rate) can be compared to the crude rate or to any other similarly standardized rate. Indirect standardization: Expected number of deaths can be compared to the number of actual deaths with the SMR.