On Sat, Dec 15, 2012 at 10:04 PM, Omphalodes Verna <
omphalodes.verna@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

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