In the numerator of the relative risk, the incidence in the exposed group is which expression?

• A. a/(a+b)
• B. b/(b+d)
• C. a/c
• D. c/(c+d)

Answer: (A)

The key idea is how we define the incidence in the exposed group when comparing risk. Incidence in the exposed group is the number of exposed individuals who develop the disease divided by all exposed individuals. In the 2x2 table, that is a divided by (a + b), where a is the exposed who become diseased and b is the exposed who remain disease-free. So the numerator for the relative risk is a/(a+b).

Why this fits: this fraction captures the probability of disease among those who were actually exposed, which is exactly what the relative risk compares to the incidence among the unexposed (c/(c+d)).

For context, b/(b+d) would be the proportion of exposed individuals who do not have the disease, not the incidence; c/(c+d) is the incidence in the unexposed group; a/c would mix diseased counts between groups in a way that isn’t an incidence measure.