And I figured it out, sorry to bother the list. The normal approximation I was using is not accurate in the presence of ties.
Sam On Thu, Oct 6, 2011 at 10:56 AM, Sam Stewart <rhelp.st...@gmail.com> wrote: > So I checked it with the wilcox_test in the coin library, and got the > same result. That makes me more confident that I made a mistake, but > still doesn't help me find it > > d = > data.frame(value=c(dropouts,remain),group=c(rep("dropout",length(dropouts)),rep("remain",length(remain)))) > wilcox_test(value~group,data=d) > > Sam > > On Thu, Oct 6, 2011 at 10:35 AM, Sam Stewart <rhelp.st...@gmail.com> wrote: >> Hello List, >> >> I'm trying to prepare some lecture notes on non parametric methods, >> and I can't manually reproduce the results of the wilcox.test function >> for ordinal data. >> >> The data I'm using are from David Howell's website, available here >> >> http://www.uvm.edu/~dhowell/StatPages/More_Stuff/OrdinalChisq/OrdinalChiSq.html >> >> If I run the wilcox.test function on the data I get a p-value of >> .0407, but when I do it myself I get a p-value of 0.0530. It's not so >> much the jump across 0.05, but the fact that I thought I knew what the >> function was doing. >> >> I know from the R help page that there is some controversy about how >> exactly to calculate the test statistic, but that's not what is >> causing the problem, as I can get the same W value. Am I calculating >> the test statistic incorrectly? >> >> Thanks, sample code below >> Sam Stewart >> >> #Ordinal example >> dropouts = c(rep(0,25),rep(3,10),rep(2,9),rep(1,13),rep(4,6)) >> remain = c(rep(0,31),rep(3,2),rep(2,6),rep(1,21),rep(4,3)) >> tab2 = rbind(table(dropouts),table(remain)) >> ordTest = wilcox.test(x=dropouts,y=remain,correct=FALSE,exact=FALSE) >> cumsum(colSums(tab2)) >> W = >> max(c(sum(rank(cbind(dropouts,remain))[1:length(dropouts)]),sum(rank(cbind(dropouts,remain))[-(1:length(dropouts))]))) >> n1 = length(dropouts) >> n2 = length(remain) >> testStat = (S-n1*(n1+n2+1)/2)/(sqrt(n1*n2*(n1+n2+1)/12)) >> 2*(1-pnorm(testStat)) >> > ______________________________________________ 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.