In a case-control study of oral contraceptives and venous thrombosis, investigators match controls so tightly on age, obesity, and family history of thrombophilia that these variables can no longer be evaluated, and the measured odds ratio moves toward the null. This phenomenon is called:

• A Recall bias
• B Overmatching ✓
• C Neyman's bias
• D Confounding by indication

Explanation

Overmatching occurs when controls are matched on factors linked to the exposure itself or when matching consumes variables that should be analyzed as independent risk factors. It wastes statistical power, makes the effect of matched variables unassessable, and can bias the odds ratio toward the null. Neyman's bias refers to survival-related case selection, which is unrelated here.

Reference: Gordis Epidemiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.