## **Core Concept**
Relative risk (RR), also known as the risk ratio, is a measure used in epidemiology to compare the risk of a certain event (such as developing a disease) occurring in one group with the risk of the same event occurring in another group. It is a ratio of the probability of an event occurring in the exposed group versus the non-exposed group.

## **Why the Correct Answer is Right**
The formula for relative risk is: RR = [a / (a + b)] / [c / (c + d)], where 'a' represents the number of individuals with the event in the exposed group, 'b' represents the number of individuals without the event in the exposed group, 'c' represents the number of individuals with the event in the non-exposed group, and 'd' represents the number of individuals without the event in the non-exposed group. This matches option , which correctly represents the relative risk.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option does not accurately represent the formula for relative risk. It seems to confuse the concept with odds ratio or another measure.
- **Option B:** This option also does not accurately represent the relative risk. It might be confused with a different epidemiological measure.
- **Option D:** This option seems to represent an odds ratio rather than a relative risk, given its form.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that a relative risk of 1 indicates no difference in risk between the two groups. A RR greater than 1 suggests an increased risk, and a RR less than 1 suggests a decreased risk in the exposed group compared to the non-exposed group.

## **Correct Answer:** .