In a matched pair case-control study of low birth weight and maternal anaemia, each case is matched to one control. Analysis shows 30 pairs in which the case mother was anaemic and the control mother was not, and 10 pairs in which the case mother was not anaemic and the control mother was. The matched odds ratio is:
- A 0.33
- B 3.0 ✓
- C 4.0
- D Cannot be calculated from discordant pairs alone
Correct answer: B. 3.0
Explanation
In individually matched data, concordant pairs contribute no information about the association, and the matched odds ratio is calculated as b divided by c, the two categories of discordant pairs. Here it is 30 divided by 10, giving 3.0. Using the total counts as in an unmatched 2 by 2 table would be incorrect because matching must be preserved in the analysis, typically confirmed with McNemar's test.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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