Pain scores measured on a visual analogue scale were recorded in the same 40 patients before and after physiotherapy. The differences are not normally distributed. The most appropriate test for analysing these paired observations is:

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

Explanation

Wilcoxon signed-rank test is the non-parametric analogue of the paired t-test, designed for two related measurements when differences violate normality. Mann-Whitney U compares two independent groups and Kruskal-Wallis compares three or more independent groups, so both fail the pairing requirement. Applying a paired t-test to non-normal differences risks invalid inference at this small sample size.

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

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