Which of the following statements correctly distinguishes relative risk from odds ratio?

Explanation

## Relative Risk vs Odds Ratio: Core Distinction **Key Point:** Relative risk (RR) and odds ratio (OR) are fundamentally different measures of association, each suited to different study designs and interpretations. ### Definitions **Relative Risk (RR)** - Compares the **risk (probability) of disease** in the exposed group to the risk in the unexposed group - Formula: $RR = \frac{\text{Risk in exposed}}{\text{Risk in unexposed}} = \frac{a/(a+b)}{c/(c+d)}$ - Used in **cohort studies and RCTs** where you follow groups forward in time **Odds Ratio (OR)** - Compares the **odds of exposure** in those with disease to the odds of exposure in those without disease - Formula: $OR = \frac{\text{Odds of exposure in cases}}{\text{Odds of exposure in controls}} = \frac{a/c}{b/d}$ - Used in **case-control studies** where you start with disease status and look backward ### Study Design Alignment | Study Design | Primary Measure | Why | | --- | --- | --- | | Cohort | Relative Risk | You know exposure status at baseline and follow for disease | | Case-control | Odds Ratio | You identify cases and controls, then assess past exposure | | RCT | Relative Risk | Randomization and prospective follow-up | | Cross-sectional | Either (OR preferred if prevalence >10%) | Can measure both exposure and disease simultaneously | ### Relationship Between RR and OR **High-Yield:** When disease is **rare** (prevalence <10%), the odds ratio **approximates the relative risk**. When disease is **common**, OR > RR. $$\text{In rare disease: } OR \approx RR$$ **Clinical Pearl:** This approximation is why case-control studies remain valuable for rare diseases — the OR can be interpreted as an approximate RR. ### Mnemonic **"CROC" — Cohort uses RR; cOntrol (case-control) uses OR**