## **Core Concept**
The question pertains to the concept of confidence intervals in the context of relative risk (RR) in epidemiological studies. The relative risk, also known as the risk ratio, is a measure of the ratio of the probability of an event occurring in the exposed group versus the non-exposed group. A confidence interval (CI) for RR provides a range of values within which the true RR is likely to lie. The null value for a confidence interval of relative risk refers to the value that indicates no difference in risk between the exposed and non-exposed groups.

## **Why the Correct Answer is Right**
The correct answer, 1, represents the null value for the relative risk. This is because a relative risk of 1 implies that there is no difference in risk between the exposed and non-exposed groups. If the 95% confidence interval for RR includes 1, it suggests that the observed difference in risk could be due to chance, and we fail to reject the null hypothesis that there is no association between exposure and outcome.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 0 - A relative risk of 0 would imply that there is no risk in the exposed group, which does not serve as a null value for comparing two groups.
- **Option B:** -1 - A negative relative risk does not make sense in this context, as RR is a ratio of probabilities and cannot be negative.
- **Option C:** 0.5 - While a relative risk of 0.5 would indicate a protective effect, it does not represent the null value.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that when constructing a confidence interval for relative risk, if the interval includes 1, the result is not statistically significant at the 5% level, suggesting that the observed association could be due to chance.

## **Correct Answer:** . 1