Crude rates such as incidence and attack rate summarize the effects of which two factors?

• A. Time since outbreak and population density
• B. Subgroup-specific rate and subgroup distribution
• C. Age and sex only
• D. Geographic region and climate

Answer: (B)

Crude rates reflect two components: how common the disease is within each subgroup (the subgroup-specific rate) and how the population is composed in terms of those subgroups (the subgroup distribution). The overall crude rate is a weighted average of the subgroup-specific rates, with weights given by the size of each subgroup in the population. If rates inside subgroups change, or if the population structure shifts (more people in subgroups with higher risk), the crude rate changes accordingly. This is why the two factors matched by crude rates are the subgroup-specific rates and how those subgroups are distributed in the population.

Other options point to factors that can influence disease risk or exposure but are not what crude rates summarize directly. Time or rate of exposure, geographic or climatic factors, and merely naming age and sex as the factors don’t capture both the within-subgroup risk and the population composition that together determine the crude rate.