# How to compute the power of Wilcoxon test?

I am using Wilcoxon test to compare two paired sets of data for whether their means differ. Besides the p-value, I would also like to know the power of this test. How to compute it in R? Thank you in advance.

-
One comment: The wilcoxon signed rank test, tests the hypothesis that the MEDIANS differ I do believe. udel.edu/~mcdonald/statsignedrank.html – B_Miner Sep 14 '11 at 14:08
B_Miner, no, it is for means. There is an assumption of balance around the median but the medians is not what you're comparing. – John Sep 14 '11 at 14:39
The power depends on the shape of the true distribution of the pairwise differences. You have to specify your assumption what kind of distribution this is. – caracal Sep 14 '11 at 15:34
@john - is the source I cite incorrect? – B_Miner Sep 14 '11 at 18:32
B_Miner... I did a few quick searches. An intro stats text next to me, a Vassar site, and Wikipedia support my assertion. Nevertheless, there are a couple of 'handbooks' like the one you cited that do state it's about differences between medians. I didn't read them all carefully but now I'm thinking about it and I'm wondering if it actually matters. In fact, the NIST reference I found didn't even mention means or medians. hmmmm... probably more time than I want to spend on this topic until I actually want to do one of these. – John Sep 14 '11 at 22:56