Dear R-users, If you use the exact Wilcoxon test in the coin package, I would like make you aware of that SPSS/StatXact MAY perform a round-off before doing their exact Wilcoxon-Mann-Whitney test (if you ever are unlucky enough not to use R).
I have data from two treatments and was surprised to find that SPSS (15 under Windows) and R differed in their p-values (0.167 resp. 0.172). It turns out that SPSS rounds off an entry which is equal to -0.899999999999999 to -0.9 (thus producing a tie at this position). While the former entry probably is the result of some stupid algorithmic twist in some software (take your guesses), I think that wilcox_exact in the coin package is superior by not assuming that data are to be tied when deemed close enough by some poor defined measure (but I'm biased). Try wilcox_test(delta_hba1c~as.factor(Treatment), data=U, distribution="exact") And wilcox_test(signif(delta_hba1c,3)~as.factor(Treatment), data=U, distribution="exact") Thanks to Torsten Hothorn for discussing this with me and for producing such a neat package (together with Kurt Hornik, Mark A. van de Wiel and Achim Zeileis)! Cheers, Fredrik Nilsson. My data: U a data frame with "Treatment";"delta_hba1c" 1;-1.7 2;-1.9 1;0 1;-1.9 1;-1 1;-1.2 2;-1.7 2;-1.6 1;-0.9 2;-3.5 2;-3.1 1;-1.6 2;-1.2 2;-3.8 2;-0.899999999999999 2;-1.4 2;-1.2 1;-1.3 1;-1.6 2;-1.8 1;-1.5 1;-0.7 2;-0.7 2;0 2;-1.8 1;-1.2 1;-0.3 1;-2 1;-2.3 2;-5 2;-1.3 1;-2.6 2;-1.4 2;-3.6 2;-5.6 1;-1.7 ______________________________________________ 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.
