In a study of 18 patients, 2 developed wound infection after clean surgery versus 1 of 15 controls. Two cells of the 2x2 table show expected frequencies below 5. The most appropriate test for these data is:
- A Fisher exact test ✓
- B Pearson chi-square test with Yates correction
- C McNemar test
- D Chi-square goodness of fit test
Correct answer: A. Fisher exact test
Explanation
When any expected cell frequency in a 2x2 table falls below 5, especially with small total numbers, the chi-square approximation breaks down and the Fisher exact test is the valid alternative because it computes exact probabilities from the hypergeometric distribution. Yates correction is reserved for tables where expected frequencies exceed 5 but totals are modest. McNemar applies to paired categorical data and the goodness of fit test compares observed with expected distributions, neither fitting unpaired small-sample comparison.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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