In a case-control study, how do you select controls and compute the odds ratio?

• A. Controls should come from the same source population without disease; OR = (a/c) / (b/d), where a=exposed cases, b=exposed controls, c=unexposed cases, d=unexposed controls.
• B. Controls should be randomly selected from any population; OR is calculated as (a+b)/(c+d).
• C. Controls must be diseased individuals; OR is (a+d)/(b+c).
• D. Controls should be healthier individuals from the community and OR isn't used.

Answer: (A)

In a case-control study, the essential idea is to compare how often exposure occurred among people with the disease (cases) to how often exposure occurred among people without the disease (controls) from the same population. This ensures the control group reflects the exposure distribution of the population that gave rise to the cases, avoiding selection bias.

Label the counts in a 2x2 table as: a = exposed cases, b = exposed controls, c = unexposed cases, d = unexposed controls. The odds ratio is then the odds of exposure among cases divided by the odds of exposure among controls, which is (a/c) ÷ (b/d) = ad/bc. This formula captures how much more (or less) likely cases were to have been exposed compared with controls, and it uses exposure odds rather than risk because case-control studies don’t provide incidence data.

Why this answer is best: it specifies that controls come from the same source population without disease to ensure valid comparison, and it gives the correct odds-ratio computation for a case-control design. Other options either pick inappropriate controls, mix in diseased individuals, or misstate how the measure is calculated, which would lead to biased or meaningless results in this study design.