Which statement correctly matches the measure to its typical interpretation in cohort data?

• A. RR is the absolute difference in risk; rate ratio is the ratio of hazards; RD compares probabilities; Use all three interchangeably.
• B. Rate ratio compares incidence rates; use it when person-time varies.
• C. RR is the same as rate ratio; RD is the same as RR; Use rate ratio when follow-up is fixed.
• D. All three measures are interchangeable; choose based on convenience.

Answer: (B)

In cohort data, it’s important to distinguish between measures that use probability of disease over time and those that use incidence rate per person-time. A rate ratio compares incidence rates, which are the number of new cases divided by the total person-time at risk. This makes the rate ratio the right choice when follow-up times differ across individuals or groups, because it accounts for how long people were actually observed.

Relative risk (risk ratio) looks at the probability of developing disease over a fixed period, assuming comparable follow-up, and the risk difference is the absolute difference in those probabilities. These are not interchangeable with the rate ratio, which is why the statement that rate ratio should be used when person-time varies is the best interpretation.

For example, if one group contributes more person-time but similar numbers of cases, the incidence rate treatment will reflect that difference, yielding a meaningful rate ratio.