A researcher tests whether a new vaccine reduces the incidence of measles by comparing proportions between vaccinated and unvaccinated groups using a chi-square test. The degrees of freedom for a 2×2 contingency table used in this analysis is:
- A 2
- B 1 ✓
- C 3
- D 4
Correct answer: B. 1
Explanation
Degrees of freedom for a chi-square test on a contingency table = (rows − 1) × (columns − 1). For a 2×2 table: (2−1) × (2−1) = 1 × 1 = 1. This is the standard 2×2 chi-square for comparing two proportions such as vaccine vs. no vaccine and disease vs. no disease. Yates' continuity correction may be applied when expected cell counts are small, but degrees of freedom remain 1.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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