A researcher constructs a 2x2 table comparing an exposure with a rare disease outcome. Several cells have expected frequencies below 5 despite a total sample size of 28. Which statistical test should be applied instead of the standard chi-square test?
- A Yates corrected chi-square test
- B McNemar test
- C Fisher exact test ✓
- D Student t-test
Correct answer: C. Fisher exact test
Explanation
The chi-square approximation becomes unreliable when any expected cell frequency falls below 5 in a 2x2 table, particularly with small totals. Fisher exact test calculates the exact probability from the hypergeometric distribution and is the standard replacement. Yates correction is applied when expected values lie between 5 and 10 but is inadequate below 5. McNemar test is reserved for paired categorical data.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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