In a case-control study design, which describes how cases and controls are defined?

• A. Cases defined by exposure status; controls defined by exposure absence
• B. Random sampling
• C. Cases defined by outcome status; controls defined by absence of outcome
• D. Cases defined by geographic location

Answer: (C)

In a case-control study, groups are defined by disease or outcome status. Cases are individuals who already have the outcome of interest, while controls are individuals who do not have that outcome. After forming these groups, researchers look back to see who was exposed in the past, which helps estimate the association between exposure and the outcome.

The other ideas don’t define the groups correctly: defining cases by exposure would create groups based on past exposure rather than current outcome; random sampling describes how participants are chosen rather than how cases and controls are defined; and using geographic location isn’t how you distinguish cases from controls, though location might be used for matching or sampling frames.