In a matched case-control study of oral contraceptives and venous thrombosis, 100 case-control pairs are analysed. In 30 pairs only the case used oral contraceptives, in 12 pairs only the control used them, and in the remaining pairs both members had identical exposure status. The matched odds ratio is:

• A 2.5 ✓
• B 0.42
• C 1.5
• D Cannot be calculated without knowing concordant pairs

Explanation

In matched-pair analysis, concordant pairs where both members share exposure status carry no information about the association, so they drop out of the calculation. The matched odds ratio equals the number of pairs in which only the case was exposed divided by the number of pairs in which only the control was exposed: 30/12 equals 2.5. Option B is the reciprocal, arising from inverting the ratio. Concordant pair counts are genuinely unnecessary, which makes option D wrong.

Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.

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