In a case-control study design, how are study sites selected?

• A. Based on exposure status
• B. Based on outcome status (crash or no crash)
• C. Random sampling
• D. Based on age groups

Answer: (B)

In a case-control study, you form groups by the outcome. Study sites are chosen so you can identify individuals who have the outcome of interest (cases) and those who do not have the outcome (controls) from a common source population. This setup lets you look back at prior exposures and compare how often exposures occurred in cases versus controls, helping to reveal factors associated with the outcome. Selecting by exposure status would bias the design, since exposure is what you’re studying rather than the basis for forming the groups. Random sampling of sites wouldn’t guarantee you’d capture distinct outcome groups, and choosing by age groups doesn’t define cases and controls by outcome either.