On Oct 2, 2013, at 8:33 AM, Filipe Correia wrote: > Hello everyone, > > I'm having some trouble interpreting the results of a Wilcoxon > (Mann-Whitney U) test. Hope you can help. > > This is the R script that I am running: > > a <- c(1, 1, 1, 1, 1, 1, 1, 1, 3, 1, 1, 1, 2, 1, 5, 1, 1, 1, 3, 1, 1, > 1, 1, 1, 1, 3, 1, 1) > b <- c(1, 2, 1, 1, 2, 3, 2, 2, 1, 2, 1, 1, 1, 2) > wilcox.test(a, b, alternative="t", mu=0, exact=FALSE, paired=FALSE) #1st > wilcox.test(a, b, alternative="l", mu=0, exact=FALSE, paired=FALSE) #2nd > wilcox.test(a, b, alternative="g", mu=0, exact=FALSE, paired=FALSE) #3rd > > ... and it's returning: > > Wilcoxon rank sum test with continuity correction data: a and b > W = 145, p-value = 0.08969 > alternative hypothesis: true location shift is not equal to 0 > > Wilcoxon rank sum test with continuity correction data: a and b > W = 145, p-value = 0.04485 > alternative hypothesis: true location shift is less than 0 > > Wilcoxon rank sum test with continuity correction data: a and b > W = 145, p-value = 0.9582 > alternative hypothesis: true location shift is greater than 0 > > The null hypothesis is that the populations are equivalent (mu=0). The > alternative hypothesis are that they differ, with the 2nd and 3rd runs > of the test above considering respectively that a<b and b>a. Plus, I'm > considering an alfa of 0.05. > > My issue is that from the first run I could not conclude that there > was a difference between the two populations (0.08969>0.05), but the > second run leads me to think that a<b (because 0.04485<0.05).
This is not an appropriate question for R-help. You should ask your statistics instructor or pose the question in a venue where explanations of purely statistical problems are on-topic. (Just because you used R to get results that confused you does not make this an R-help-problem.) > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html -- David Winsemius Alameda, CA, USA ______________________________________________ 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.
