In a smoking cessation study, 100 subjects are classified as smokers or non-smokers before counselling and again after 6 months. The investigator wants to test whether the proportion of smokers changed significantly. The appropriate test is:
- A McNemar's test ✓
- B Pearson chi-square test
- C Fisher's exact test
- D Z-test for two independent proportions
Correct answer: A. McNemar's test
Explanation
McNemar's test is designed for paired nominal data, where each subject serves as his own control and is classified twice, as in before-and-after studies. The ordinary chi-square test and the z-test for proportions assume two independent samples, which is not true here because the same 100 subjects contribute both measurements. Fisher's exact test also applies to independent groups in a single 2x2 classification.
Reference: Mahajan's Methods in Biostatistics for Medical Students and Research Workers, 9th ed.
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