Relative risk is define as ?
**Core Concept**
Relative risk is a measure used in epidemiology to compare the risk of a certain outcome or disease between two groups, typically an exposed group and a non-exposed group. It is a ratio of the probability of an event occurring in the exposed group to the probability of the event occurring in the non-exposed group.
**Why the Correct Answer is Right**
Relative risk is calculated as the ratio of the incidence rate of the outcome in the exposed group to the incidence rate of the outcome in the non-exposed group. This measure helps to quantify the association between a risk factor and an outcome, and it is widely used in clinical and public health research. The formula for relative risk is: RR = (Incidence rate in exposed group) / (Incidence rate in non-exposed group). For example, if the incidence rate of a disease is 10% in a group exposed to a certain risk factor and 5% in a group not exposed, the relative risk would be 2, indicating that the risk of the disease is twice as high in the exposed group.
**Why Each Wrong Option is Incorrect**
**Option A:** Incidence rate is a measure of the number of new cases of a disease occurring in a population over a specified period, but it is not a measure of the risk of the disease. Therefore, it is not the correct definition of relative risk.
**Option B:** Odds ratio is a measure of association between an exposure and an outcome, but it is not the same as relative risk. Odds ratio is calculated as the ratio of the odds of the outcome in the exposed group to the odds of the outcome in the non-exposed group.
**Option C:** Absolute risk reduction is a measure of the difference in risk between two groups, but it is not a ratio of risks. Therefore, it is not the correct definition of relative risk.
**Clinical Pearl / High-Yield Fact**
When interpreting relative risk, it's essential to consider the absolute risk reduction, which is the difference in risk between the exposed and non-exposed groups. A small relative risk may be clinically significant if the absolute risk reduction is large.
**Correct Answer: C. Absolute risk reduction is a measure of the difference in risk between two groups, but it is not a ratio of risks. Therefore, it is not the correct definition of relative risk.**