You are working as an analyst at the local health department. Your boss has just

Question

You are working as an analyst at the local health department. Your boss has just come across some disease estimates that are concerning to him. He reports to you that the standardized mortality ratio (SMR) for ovarian cancer in Colorado is 3.7, while it is only 1.3 in New Mexico. Frantic, he exclaims “Colorado has 2.8 times the deaths from ovarian cancer compared to New Mexico, something must be done!” Is his interpretation of this information correct? Yes, the SMR accounts for differences in the age distribution between New Mexico and Colorado by comparison to a standard population, therefore you can conclude that Colorado has more deaths from ovarian cancer than New Mexico. No, you cannot compare two SMRs to each other. You can only determine if the observed number of deaths in the study population is higher than the expected number of deaths if the study population had the same mortality experience of the comparison population. No, you cannot not draw any conclusions without knowing if New Mexico and Colorado were adjusted to the same standard population.

Explanation / Answer

No, you cannot compare two SMRs to each other. You can only determine if the observed number of deaths in the study population is higher than the expected number of deaths if the study population had the same mortality experience of the comparison population.

The lower SMR in New Mexico than in Colorado does not necessarily imply that its mortality rates by age are lower. This is because the expected numbers of deaths are calculated by applying agespecific rates to the age distribution of each area. Consequently where two areas have identical death rates in every age group, but different population age structures, their SMRs may differ. Hence, if the ratio of death rates in each area varies with age and the population structures are also very different, then the SMRs for these areas may provide a misleading summary comparison between the areas affected.

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