A researcher constructs a 2x2 table comparing an exposure with a rare disease outcome. Several cells have expected frequencies below 5. Which test should be applied?

• A Standard Pearson chi-square test
• B Student t-test
• C Mann-Whitney U test
• D Fisher's exact test ✓

Explanation

The chi-square test is valid only when expected cell frequencies are adequately large, conventionally all cells above 5 in a 2x2 table. When any expected value falls below 5, Fisher's exact test is used because it calculates the exact probability from the hypergeometric distribution without relying on large-sample approximation. Mann-Whitney U is for ordinal or non-normal continuous data, and the t-test is for means.

Reference: Mahajan's Methods in Biostatistics for Medical Students and Research Workers, 9th ed.

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