Distinguish relative risk and odds ratio.

• A. Relative risk compares probabilities; odds ratio compares odds; OR approximates RR when disease is rare.
• B. Relative risk and odds ratio are the same measure.
• C. Relative risk is used for cohort studies; odds ratio for case-control only.
• D. Odds ratio is always larger than relative risk.

Answer: (A)

The main idea is how these two measures describe association differently and how they relate when the outcome is rare. Relative risk looks at probabilities: it is the risk of disease in the exposed group divided by the risk in the unexposed group. The odds ratio, on the other hand, compares odds: the odds of having the disease given exposure divided by the odds of having the disease without exposure (equivalently the cross-product ratio ad/bc from a 2x2 table).

When the disease is rare, odds and probabilities are very close, so the odds ratio ends up very close to the relative risk. For example, if the risk is 1% with no exposure and 2% with exposure, the relative risk is 2, and the odds ratio is about 2.02—nearly the same. But when the disease is more common, the odds ratio tends to overstate the relative risk and becomes harder to interpret as a direct risk estimate.

In terms of study design, relative risk is most straightforward to interpret in cohort studies where you can observe incidence over time, while the odds ratio is often used in case-control studies (where you fix the number of cases and controls) and is routinely produced by logistic regression.

So, relative risk compares probabilities, odds ratio compares odds, and the odds ratio approximates the relative risk when the disease is rare.