In a standardized mortality ratio (SMR) study of miners, 60 deaths from respiratory disease are observed while only 40 would be expected based on national age-specific rates. The SMR is 150%. Which interpretation is correct?

• A Miners have a 150% higher mortality than the general population
• B The attributable risk percent among miners is 150%
• C Observed deaths exceed expected deaths by 50% ✓
• D The relative risk cannot be calculated from an SMR

Explanation

SMR equals observed deaths divided by expected deaths, expressed as a percentage. Here 60 divided by 40 gives 1.5, or 150%, meaning mortality is 50% above expectation, a 50% excess. Option A commits the common error of equating an SMR of 150% with a 150% increase; a 150% increase would require an SMR of 250%. An SMR is itself an indirectly standardized relative measure, so a relative estimate clearly can be derived.

Reference: Gordis Epidemiology, 6th ed.

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