The short answer is whichever version you want since they are the same. Look at the t-test results, they are also different. In one t is positive and in the other it is negative. That is just a result of whether the smaller mean is subtracted from the larger mean or vice versa. The same is happening with Wilcoxon (see http://en.wikipedia.org/wiki/Mann-Whitney-Wilcoxon_test ) for the specific equations.
------------------------------------- David L Carlson Associate Professor of Anthropology Texas A&M University College Station, TX 77840-4352 -----Original Message----- From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On Behalf Of Alastair Potts Sent: Thursday, July 18, 2013 4:18 AM To: r-help@r-project.org Subject: [R] Orders of levels affecting wilcox.test() output Good day all, My first posting to this list. It looked like the best place to post this question. When running the wilcox.test(), I noticed that the output values change if you change the ordering of the levels (example below which includes a t.test for comparison). I think this has something to do with the change in ranking order, but this doesn't make much sense as I would expect the ranking in this case to remain constant as there are no ties. So my questions are: a) is this correct (i.e. not a bug)?, and b) if it is, then which values should I report? x <- cbind(data.frame((c(rep("A",50),rep("B",50)))),runif(100)) colnames(x) <- c("a","b") wilcox.test(b~a,data=x) t.test(b~a,data=x) x$a <- relevel(x$a,ref="B") wilcox.test(b~a,data=x) t.test(b~a,data=x) Thank you in advance for your time, Regards, Alastair [[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. ______________________________________________ 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.
