A researcher constructs a 2x2 contingency table comparing an exposure and a disease outcome in 25 subjects. The calculated expected frequency in one of the cells is 3. Which statistical test should be applied?
- A Chi-square test without correction
- B Student's t-test
- C Fisher's exact test ✓
- D Mann-Whitney U test
Correct answer: C. Fisher's exact test
Explanation
The chi-square approximation becomes unreliable when any expected cell frequency in a 2x2 table falls below 5, particularly with small total numbers. Fisher's exact test calculates the exact probability of the observed arrangement under the null hypothesis and is valid regardless of expected counts. Yates' correction only improves chi-square accuracy marginally but does not rescue tables with very small expected values, so it is not the answer here.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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