# Wilcoxon-Mann-Whitney U value: outcomes from different stat packages

4 messages
Open this post in threaded view
|
Report Content as Inappropriate

## Wilcoxon-Mann-Whitney U value: outcomes from different stat packages

 Given this example #start code a<-c(0,70,50,100,70,650,1300,6900,1780,4930,1120,700,190,940,      760,100,300,36270,5610,249680,1760,4040,164890,17230,75140,1870,22380,5890,2430) b<-c(0,0,10,30,50,440,1000,140,70,90,60,60,20,90,180,30,90,      3220,490,20790,290,740,5350,940,3910,0,640,850,260) wilcox.test(a, b, paired=FALSE) #sum of rank for first sample sum.rank.a <- sum(rank(c(a,b))[1:29]) #sum of ranks assigned to the group a W1<- sum.rank.a - (length(a)*(length(a)+1)) / 2 W1 U1 <- length(a)*length(b)/2-W1 U1 #sum of ranks for second sample sum.rank.b <-sum(rank(c(a,b))[30:58]) #sum of ranks assigned to the group b W2 <- sum.rank.b - (length(b)*(length(b)+1)) / 2 W2 U2 <- length(a)*length(b)/2-W2 U2 #end code And given the fact that: - in the note of R Wilcox.test is clearly stated: “ The literature is not unanimous about the definitions of the Wilcoxon rank sum and Mann-Whitney tests. The two most common definitions correspond to the sum of the ranks of the first sample with the minimum value subtracted or not. R subtracts [….], giving a value which is larger by m(m+1)/2 for a first sample of size m” - as result of the same test performed with different stat packages (i.e. STATISTICA and PAST) I’ve got an U value of 200.5 as in W2 (see my script) with the same p-value What can I conclude regarding STATISTICA and PAST packages?... are they giving W2 (see my script) instead of U? A crucial point is that the variant of the algorithm used for computation by the packages is very rarely indicated in the output or documented in the help facility and the manuals. See also this link (I’ve found after a long meandering on the web) about the comparison of “wilcoxon mann whitney” u test outcomes from different stat packages: http://www.jstor.org/discover/10.2307/2685616?uid=3738296&uid=2129&uid=2&uid=70&uid=4&sid=47699045750617  Any of you have faced the same type of issues? Or am I completely wrong? maxbre
Open this post in threaded view
|
Report Content as Inappropriate

## Re: Wilcoxon-Mann-Whitney U value: outcomes from different stat packages

Open this post in threaded view
|
Report Content as Inappropriate