Question

Use the following figure to answer questions 12-14: Figure 3b (British J Derm 2008) comes from a randomized trial of DHA versus a placebo pill for treating eczema. The figure shows boxplots of the eczema severity score, SCORAD, at baseline and week 8 for each group. SCORAD score is not normally distributed and the sample size is small.

The image shows 4 box plots. Box Plot A (DHA, Week 0) Median 32, lower quartile 29 and upper quartile 44 (minimum 18 and maximum 49); Box Plot B (DHA, Week 8) Median 28, lower quartile 23 and upper quartile 34 (minimum 18 and maximum 52); Box Plot C (Control, Week 0) Median 36, lower quartile 30 and upper quartile 45 (minimum 18 and maximum 65); Box Plot D (Control, Week 8) Median 34, lower quartile 25 and upper quartile 38 (minimum 13 and maximum 55)

Reproduced with permission from: Koch C, Dölle S, Metzger M, et al. Docosahexaenoic acid (DHA) supplementation in atopic eczema: a randomized, double-blind, controlled trial. Br J Dermatol 2008;158:786-792.

QUESTION 12

12. What test should be used to determine whether DHA is a better treatment than control for treating eczema (SCORAD is not normally distributed)?

A t-test that compares A and B. A Wilcoxon sign-rank test that compares B and D. Wilcoxon sign-rank tests comparing A with B and C with D. A Wilcoxon sum-rank test that compares the difference between A and B with the difference between C and D. A chi-square test that compares B and C

Answer 1

A Wilcoxon sum-rank test that compares the difference between A and B with the difference between C and D.