Worik R wrote: > I have been examining the Mann-Whitney test closely. And there are two > features of the R implementation that puzzles me. The test statistic is > reported as "W" and depends on the order of the arguments to the function. .... > Why calculate U, or W, at all?
Did you read the note on the help page and follow up on the reference therein? Those test statistics are all simple linear transformations of the sum of the ranks in one group or the other, and it is purely a matter of convention which one to report since they are all equivalent (since the sum of ranks over both groups is a constant, etc.). However, it is conventional to report some statistic for the test, and R chooses one of them. Textbooks sometimes switch the groups to get the smallest sample size in the first group, for reasons of table lookup (and, seemingly, because people are less easily confused than computers...) -- Peter Dalgaard Center for Statistics, Copenhagen Business School Phone: (+45)38153501 Email: pd....@cbs.dk Priv: pda...@gmail.com ______________________________________________ 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.