Which of the following is a case-control study?

• A. It is relatively inexpensive compared with most other epidemiologic study designs
• B. Patients with the disease are compared with persons without the disease
• C. Incidence rates may be computed directly
• D. Assessment of past exposure may be biased

Answer: (C)

In a case-control study, you start with two groups based on disease status: those with the disease and those without, and then look back to compare their prior exposures. This outcome-based selection is what defines the design. Because you’re choosing participants by disease status, you can’t directly compute incidence or risk in the population from the study data; instead, you typically estimate the association between exposure and disease using an odds ratio. So the defining idea is the comparison between cases (disease) and controls (no disease) to assess past exposure. While case-control studies are often relatively quick and inexpensive and can be subject to recall bias in assessing exposure, these points are about limitations or practical aspects rather than what makes the design itself.