Why can't two SMRs be directly compared?

• A. They are not adjusted for age.
• B. They measure different diseases.
• C. Each SMR is adjusted to its own standard population, making direct comparison invalid.
• D. They require identical study populations.

Answer: (C)

The key idea is standardization: SMR is a ratio that makes mortality comparable by using a standard population to adjust for age. An SMR compares observed deaths in a study group to the number of deaths that would be expected if that same group had the age-specific death rates of a chosen standard population. Because the standard population sets those age-specific rates and also defines the age structure used for the comparison, using different standard populations changes the expected counts and therefore changes the SMR itself. So two SMRs computed with different standards aren’t directly comparable unless you recalculate them using the same standard population (or convert them to a common standard).

Age adjustment is inherent in SMR, so the issue isn’t whether age is considered but which standard is used. SMRs can come from different populations or diseases, and they don’t require identical study populations for estimation; the caveat is that the standard used for comparison must be consistent.