What are the common measures of association in cohort studies?

• A. Relative risk, hazard ratio, risk difference, and odds ratio for rare outcomes.
• B. Odds ratio, prevalence ratio, rate difference, and hazard difference.
• C. Relative risk, incidence rate ratio, risk difference, and sometimes odds ratio for rare outcomes.
• D. Hazard ratio, risk ratio, risk difference, and ratio of means.

Answer: (C)

In cohort studies, you’re comparing how often an outcome occurs in those exposed versus those not exposed, so the measures you’d use fall into two broad kinds: relative measures that compare the likelihood of the outcome between groups, and absolute or rate-based measures that quantify the actual amount of risk or the rate of occurrence. The relative risk (risk ratio) is the most direct and intuitive way to express how much more (or less) likely the outcome is in the exposed group compared with the unexposed group. When follow-up times differ or you’re using person-time data, the incidence rate ratio extends this idea by comparing incidence rates (events per person-time) rather than simple probabilities.

The risk difference provides the absolute excess (or deficit) risk due to exposure, which is often crucial for understanding public health impact in concrete terms. And while odds ratios are more commonly associated with case-control studies or logistic models, they’re sometimes used in cohort analyses when the outcome is rare; in that situation the odds ratio mirrors the relative risk closely enough to be informative.

Other measures like prevalence ratio are more suited to cross-sectional data, and hazard ratio is a time-to-event measure from survival analysis that fits within cohort studies but is not listed in this particular set. The combination of relative risk, incidence rate ratio, risk difference, and the occasional odds ratio for rare outcomes captures the core ways to quantify association in standard cohort analyses.