ABSTRACT
Clinical investigations often involve data in the form of ordered categories — e.g., “worse,” “unchanged” “improved,” “much improved.” Comparison of two groups when the data are of this kind should not be done by the chi-square test, which wastes information and is insensitive in this context. The Wilcoxon-Mann-Whitney test provides a proper analysis. Alternatively, scores may be assigned to the categories in order, and the t-test applied. We demonstrate both approaches here.
Sometimes data in ordered categories are reduced to a 2 × 2 table by collapsing the high categories into one category and the low categories into another. This practice is inefficient; moreover, it entails avoidable subjectivity in the choice of the cutting point that defines the two super-categories. The Wilcoxon-Mann-Whitney procedure (or the t-test with use of ordered scores) is preferable.
A survey of research articles in Volume 306 (1982) of the New England Journal of Medicine shows many instances of ordered-category data and no instance of analysis by the preferred methods presented here. Seven years later, Volume 321 (1989) shows several instances of analysis by the preferred methods that use the ordering.
