Which of the following is a key disadvantage of case-control studies compared to cohort studies?

Explanation

## Key Disadvantages of Case-Control Studies **Key Point:** The fundamental limitation of case-control studies is that they are **retrospective in nature** — they begin with the disease outcome and look backward to exposure history. This design prevents direct calculation of **relative risk**. ### Why Relative Risk Cannot Be Directly Calculated In a case-control study: - The researcher starts with **cases** (people with disease) and **controls** (people without disease) - The researcher then determines **past exposure status** in each group - The study does NOT follow people forward from exposure to disease occurrence Because the study does not begin with exposure groups and follow them to disease, **incidence rates cannot be calculated**. Without incidence rates, relative risk (which is the ratio of incidence rates) cannot be directly computed. ### What CAN Be Calculated in Case-Control Studies **Odds Ratio (OR)** is directly calculated: $$OR = \frac{a \times d}{b \times c}$$ where: - a = exposed cases - b = unexposed cases - c = exposed controls - d = unexposed controls The odds ratio approximates relative risk when the disease is **rare** (prevalence < 10%). ### Comparison Table | Feature | Cohort Study | Case-Control Study | |---------|--------------|-------------------| | **Direction** | Prospective (exposure → disease) | Retrospective (disease → exposure) | | **Relative Risk** | Directly calculated ✓ | Cannot be calculated ✗ | | **Odds Ratio** | Can be calculated | Directly calculated ✓ | | **Time to Results** | Longer | Shorter | | **Cost** | Higher | Lower | | **Recall Bias** | Minimal | Significant | **High-Yield:** When a case-control study reports "relative risk," it is actually reporting the **odds ratio** as an approximation of relative risk. This is acceptable only for rare diseases. **Warning:** Do not confuse "inability to calculate RR" with other disadvantages like cost or follow-up duration. Cost and time are advantages of case-control studies, not disadvantages.