In a case-control study of oral contraceptives and venous thrombosis, investigators match controls to cases on use of anticoagulant clinic services, a factor closely tied to both oral contraceptive prescription and thrombosis detection. The resulting odds ratio is markedly attenuated compared with prior literature. The most likely explanation is:

• A Recall bias in cases
• B Differential misclassification of outcome
• C Berkson's bias
• D Overmatching ✓

Explanation

Overmatching occurs when controls are matched on a variable linked to the exposure or on a link in the causal pathway, which artificially forces similarity between cases and controls and drives the odds ratio toward the null, reducing statistical power without removing real confounding. Berkson's bias is hospital-based selection bias, and recall bias concerns differential reporting of exposure, neither of which matches the mechanism described.

Reference: Gordis Epidemiology, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.