On 10/29/2010 06:24 AM, Steven McKinney wrote: >> > wilcox.test(as.integer(grade) ~ sex, data = gradesbysex) > Wilcoxon rank sum test with continuity correction > > data: as.integer(grade) by sex > W = 4.5, p-value = 0.2695 > alternative hypothesis: true location shift is not equal to 0 > > Warning message: > In wilcox.test.default(x = c(3L, 2L, 1L, 1L, 1L, 2L), y = c(2L, : > cannot compute exact p-value with ties > > You can break the ties by jittering the data. Each jitter will of course > produce different tie breakers. A few repeats of the test, or a loop and > some summaries of the outcomes, will give you an idea of the > "average" result.
I wouldn't bother with that. The p-value is based on the correct covariance matrix of the rank sums, tie-breaking just adds noise to the analysis. If you really want an exact p-value, package exactRankTests is the ticket. (Or, if there is really only 3 females in 9 students, you can get ambitious and set up the permutation distribution by enumerating the choose(9,3)=84 possible outcomes.) -- 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.
