Indirect standardization is used when study-specific age-specific rates are unstable or unavailable. Which approach describes how it is performed?

• A. Use study population's crude rates adjusted by age
• B. Apply standard population's age-specific rates to the study population's age distribution to compute expected cases
• C. Directly compare age-specific rates between populations
• D. Apply standardization by adjusting all rates to a common scale

Answer: (B)

Indirect standardization adjusts for differences in age structure when the study’s own age-specific rates are unstable or unavailable. The key idea is to take the age-specific rates from a standard population and apply them to the study population’s age distribution to estimate how many cases would be expected if the study had the same age-specific risk as the standard. You calculate expected cases by multiplying the standard rate in each age group by the number of people in that age group in the study population and summing across all ages. The observed cases in the study are then compared to this expected count to yield a ratio (the standardized incidence ratio). This approach is used precisely because the study’s own age-specific rates are unreliable or unavailable, so external standard rates are used to estimate what would be expected.