Pain scores on an ordinal scale are recorded before and after physiotherapy in the same 20 patients, and the differences are markedly non-normal. Which test should replace the paired t-test?

• A Spearman rank correlation
• B Mann-Whitney U test
• C Kruskal-Wallis test
• D Wilcoxon signed-rank test ✓

Explanation

The Wilcoxon signed-rank test is the non-parametric analogue of the paired t-test for matched or before-after designs when differences are not normally distributed. It ranks the absolute differences and analyses their signs. Mann-Whitney U handles two independent unpaired groups, Kruskal-Wallis extends this to three or more groups, and Spearman assesses association between two variables rather than a difference between paired measurements.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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