Which are the three primary sources of bias in case-control studies?

• A. Group selection; measurement of exposure; presumed temporal relations.
• B. Selection bias only.
• C. Recall bias only.
• D. Observer bias in outcome assessment.

Answer: (A)

In case-control studies, bias tends to arise from three main areas: who is included in the study, how exposure is determined, and the timing of exposure relative to disease onset.

Group selection bias happens when cases and controls come from populations that aren’t comparable or when the control group doesn’t reflect the exposure distribution of the source population that produced the cases. This can make the exposure difference between groups misleading and distort the association you’re trying to estimate.

Measurement of exposure bias occurs when exposure status is recorded inaccurately or differently between cases and controls. This includes recall bias, where diseased individuals may remember past exposures differently than non-diseased individuals, leading to differential misclassification that can inflate or weaken the observed association.

Presumed temporal relations refer to the challenge of establishing that the exposure occurred before the disease in a retrospective design. If temporality is misinterpreted—such as exposure reporting being influenced by disease, or disease onset affecting exposure recall—the inferred sequence can bias the results.

Those three areas capture the principal avenues through which bias can creep into case-control studies. Other options often cover only a single facet (like recall or observer bias) or point to outcome assessment rather than exposure, and don’t address all major sources of bias together.