On Sat, Dec 15, 2012 at 10:04 PM, Omphalodes Verna < omphalodes.ve...@yahoo.com> wrote:
> Dear list! > > I work with multiple Kruskal-Wallis test (kruskalmc, package pgirmess), > which evaluates differences in medians among groups (5 groups). A result of > a test is significant differences among some groups, while median values > are the same for 4 groups (using tapply). Why? > > The Kruskal-Wallis test *doesn't* evaluate differences in medians, so there is quite likely nothing wrong in a formal sense. However, this does suggest that your groups may not be stochastically ordered, which means the results of the Kruskal-Wallis test could be quite misleading. I'd suggest that you at least look at pairwise Wilcoxon tests to make sure the direction agrees with what the Kruskal-Wallis test implies. Box plots might also be a good idea. Or, if you really want differences in medians, look at differences in medians. A permutation test or a bootstrap confidence interval is probably the best way to do this. -thomas -- Thomas Lumley Professor of Biostatistics University of Auckland [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.