Which of the following is a fundamental disadvantage of case-control studies compared to cohort studies when investigating the association between an exposure and disease?

Explanation

## Fundamental Limitation of Case-Control Studies **Key Point:** Case-control studies cannot **directly calculate relative risk** because the study design begins with disease status (cases vs. controls) rather than exposure status, making the denominator (source population at risk) unknown. ### Why Case-Control Studies Cannot Calculate RR Directly ```mermaid flowchart TD A[Case-Control Study Design]:::outcome --> B[Start with Disease Status]:::outcome B --> C[Identify Cases with Disease]:::action B --> D[Identify Controls without Disease]:::action C --> E[Look Back at Exposure History]:::action D --> E E --> F[Calculate Odds Ratio]:::action F --> G[Cannot Calculate RR Directly]:::urgent G --> H[Source Population Unknown]:::outcome ``` ### Comparison of Measures | Study Design | Measure | Why? | |--------------|---------|------| | **Cohort** | Relative Risk | Denominator known: total at-risk population | | **Case-Control** | Odds Ratio | Denominator unknown: source population not identified | | **Cross-sectional** | Prevalence Ratio | Measures prevalence, not incidence | **High-Yield:** In case-control studies, you **cannot** calculate incidence in exposed vs. unexposed groups because you do not know the total number of exposed and unexposed individuals in the source population. The **odds ratio approximates RR** only when the disease is rare (< 10% prevalence). **Mnemonic:** **CCOR** — Case-Control studies use Odds Ratio (not RR) **Clinical Pearl:** When disease is rare, OR ≈ RR, making case-control studies efficient for rare diseases. When disease is common, OR overestimates RR. [cite:Park 26e Ch 10]