Hi, a value of 0 for the test statistic is possible. The test statistic is not just the sum of ranks, but this sum - n*(n+1)/2, where n is the number of observations of the group the rank sum is build. This statistic is equivalent to the ranks sums, since it differs only about a constant, which depends on the number of observations. Look at the following situation
> x <- 1:10 > y <- 11:20 > wilcox.test(x,y) Wilcoxon rank sum test data: x and y W = 0, p-value = 1.083e-05 alternative hypothesis: true mu is not equal to 0 When every observation of group1 is smaller than those of group2 the rank sum of the smaller group is sum(1:n1) = sum(1:10) = 10*(10+1)/2 = n1*(n1+1)/2 If you compare this to the test statistics, you'll observe that in this case the test statistic is 0. Regards, Christoph Buser -------------------------------------------------------------- Christoph Buser <[EMAIL PROTECTED]> Seminar fuer Statistik, LEO C11 ETH (Federal Inst. Technology) 8092 Zurich SWITZERLAND phone: x-41-1-632-5414 fax: 632-1228 http://stat.ethz.ch/~buser/ -------------------------------------------------------------- Liu Ting-Yuan writes: > > Hi, > > Could anyone provide the formula of the statistics which the wilcox.test > used for the two-sample rank-sum test? I got some statistics of 0 values, > but it is impossible to have 0 "rank-sum". Does the function use the > Mann-Whitney U test statistics? Thanks. > > Ting-Yuan Liu > > ______________________________________________ > [email protected] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
