What statistical measure is least appropriate when evaluating nominal variables in a clinical study?

The Mann-Whitney U test is least appropriate for evaluating nominal variables in a clinical study because it is designed specifically for ordinal or continuous data that is not normally distributed. This test compares the ranks of two independent groups, making it ideal for median differences rather than categorical data, which nominal variables represent.

In contrast, other statistical measures such as the odds ratio and relative risk are well-suited for analyzing nominal data. These measures are used for binary outcomes to assess the strength of association between two categorical variables. The chi-square test similarly evaluates the association between categorical variables by comparing observed and expected frequencies.

Thus, while the other statistical measures are appropriate for nominal data analysis, the Mann-Whitney U test does not appropriately handle the characteristics of nominal variables, which do not have a meaningful order or ranking.