Which of the following best describes the mathematical expression for Relative Risk?

Relative Risk (RR) is a measure used in epidemiology to compare the risk of a certain event occurring in two different groups. In this case, the mathematical expression for RR calculates the risk of an event in the exposed group relative to the risk in the unexposed group.

To understand why the correct choice is B, it is essential to break down the components of the expression. In the context of a 2x2 contingency table, we define:

• "a" as the number of cases in the exposed group,

• "b" as the number of non-cases in the exposed group,

• "c" as the number of cases in the unexposed group,

• "d" as the number of non-cases in the unexposed group.

The risk in the exposed group is calculated by dividing the number of cases in the exposed group (a) by the total number of individuals in the exposed group (a + b), thus giving us "a/(a + b)" as the risk of the event occurring in the exposed group. Conversely, the risk in the unexposed group is "c/(c + d)".

Relative Risk is then defined as the ratio of these two risks, expressed mathematically as