In a case-control study of oral contraceptives and stroke, investigators matched controls so closely on family history of thrombophilia that they also inadvertently matched on the causal pathway itself. The resulting loss of a demonstrable association reflects:

• A Neyman bias
• B Matching on confounders done correctly
• C Overmatching ✓
• D Misclassification bias

Explanation

Overmatching occurs when controls are matched on a variable that is part of the causal chain between exposure and outcome, or too tightly on a strong risk factor correlated with exposure. This artificially equalizes exposure distribution across cases and controls, biasing the odds ratio toward unity and masking a true effect. Neyman bias is survivor (prevalence-incidence) bias, unrelated to the matching process described here.

Reference: Gordis Epidemiology, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.