In a cohort study, what is the relative risk and how is it interpreted?

• A. Relative risk is the odds of disease among the exposed compared to unexposed; RR>1 indicates increased odds with exposure.
• B. Relative risk is the risk of disease in the unexposed relative to the exposed; RR<1 indicates protective effect.
• C. Relative risk is the ratio of the overall population risk to the exposed risk; RR=1 indicates no difference.
• D. Relative risk is the risk of the outcome in the exposed divided by the risk in the unexposed; RR>1 indicates increased risk with exposure.

Answer: (D)

In a cohort study, relative risk is the ratio of the probability of the outcome happening in the exposed group to the probability of it happening in the unexposed group. It is calculated as the risk (incidence) in the exposed divided by the risk in the unexposed. This measure works well here because cohort studies follow people over time and can directly observe how often the outcome occurs in both groups.

Interpreting it: if the relative risk is greater than 1, the exposure is associated with a higher risk of the outcome; if it’s equal to 1, there’s no difference; if it’s less than 1, the exposure is associated with a lower risk (potentially protective).

Why the other statements aren’t correct in this context: one option describes odds instead of risk, which is not the standard measure in cohort studies. Another option reverses the order of the two groups and misstates the interpretation, and another compares to the wrong reference. The described definition here—risk in the exposed divided by risk in the unexposed, with higher than 1 indicating increased risk—is the correct framing.