In a cohort study, how is relative risk calculated?

In a cohort study, how is relative risk calculated?

• A. RR is calculated as risk in unexposed divided by risk in exposed.
• B. RR is the odds of exposure among cases divided by controls.
• C. RR is calculated as risk in exposed divided by risk in unexposed.
• D. RR is the difference in risk between exposed and unexposed.

Answer: (C)

In a cohort study, relative risk compares the probability of developing the outcome between exposed and unexposed groups. It is calculated as the risk in the exposed group divided by the risk in the unexposed group, using the cumulative incidence over the study period. So you’re measuring how much the exposure changes the risk: RR = risk_exposed / risk_unexposed. For example, if 20 out of 100 exposed develop the disease and 10 out of 100 unexposed do, RR = 0.20 / 0.10 = 2, meaning the exposure doubles the risk. This contrasts with risk difference, which is an absolute change in risk, and with the odds ratio, which is used mainly in case-control studies. The first option flips the numerator and denominator, which yields the reciprocal of the correct measure.

In a cohort study, relative risk compares the probability of developing the outcome between exposed and unexposed groups. It is calculated as the risk in the exposed group divided by the risk in the unexposed group, using the cumulative incidence over the study period. So you’re measuring how much the exposure changes the risk: RR = risk_exposed / risk_unexposed. For example, if 20 out of 100 exposed develop the disease and 10 out of 100 unexposed do, RR = 0.20 / 0.10 = 2, meaning the exposure doubles the risk. This contrasts with risk difference, which is an absolute change in risk, and with the odds ratio, which is used mainly in case-control studies. The first option flips the numerator and denominator, which yields the reciprocal of the correct measure.