In a case-control study investigating whirlpool-associated infections at a health club, which group would be the most appropriate control?

• A. Ill members of the health club matched for age and sex
• B. Non-ill members of the health club matched for age and sex
• C. Health club staff members
• D. Community residents not affiliated with the health club

Answer: (B)

In a case-control study, controls should come from the same population that produced the cases and be at risk of becoming cases themselves, sharing similar exposure opportunities. Here, cases are ill health-club members with whirlpool-associated infections, so the best controls are non-ill health-club members who are matched for age and sex. They come from the same source population and have had the same chance to be exposed to the whirlpool, which allows a fair comparison of exposure to the whirlpool between cases and controls. Ill health-club members would blur the distinction between exposure and disease status, staff members may have different exposure patterns, and community residents not affiliated with the club may differ in exposure context and risk. Therefore, non-ill members of the health club matched for age and sex are the most appropriate controls.