A clinical trial compares a new topical antiseptic to placebo for preventing surgical site infection. Twenty patients are enrolled per group. The outcome is infected versus not infected. Two cells in the 2x2 contingency table have expected frequencies below 5. The most appropriate statistical test to compare the two groups is:

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

Explanation

Fisher's exact test is preferred over chi-square when expected cell frequencies fall below 5 in a 2x2 table, which commonly occurs with small sample sizes. Chi-square gives inaccurate p-values with low expected frequencies. Student's t-test requires continuous normally distributed data. Mann-Whitney U is for ordinal or skewed continuous data, not simple binary proportions.

Reference: Methods in Biostatistics by B. K. Mahajan, 7th ed.

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