On Thu, 2 Jan 2003 12:08:26 +0100, "Francois Bergeret" <[EMAIL PROTECTED]> wrote:
> hello and happy new year to the group members ! > > I have read on some references that the Wilcoxon test (or Kruskal Wallis for > more than 2 subgroups) has the assumption that the distribution of values is > symmetric aroiund the median. I'm not sure this assumption is needed. In a > book from Saporta there is a detail of the test and I do not see this > assumption. > > Is the assumption of symmetry needed for Wilcoxon or KW tests ? The rank-tests are unchanged, in fact, by any monotonic transformation (log, etc.), and that is basic about them. So, I hope that you are mis-remembering the references - The usual assumption, as I keep it in mind, is that the samples follow the *same* distribution. (Maybe someone will give the more technical statement?) As I think of them, the rank-tests save you the trouble of finding the proper transformation: a) However, the proper transformation would give the most powerful test and the most information; and b) When there is no 'proper' transformation available, the assumptions of the rank-test are probably violated, too (and, as a consequence of *this*, you probably are testing something odd, which you will regret). -- Rich Ulrich, [EMAIL PROTECTED] http://www.pitt.edu/~wpilib/index.html . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
