Dear list member,



I want to compare if the rank order is significantly different for seven
different measures. So we have same sample but different measures which
reduces the problem to a paired one sample Wilcox test if I understood the
test correctly. In constructed toy examples for my sake of understanding,
but things are not adding up. Basically, the wilcox.test() and
pairwise.wilcox.test are producing different results when they should not
(according to my understanding of course):



#take a vector

daily.intake <- c(5260,5470,5640,6180,6390,6515,

                  + 6805,7515,7515,8230,8770)



#I get desired results when I do the following

daily<-data.frame(pre=daily.intake, post=daily.intake)



#add som differences

daily[1,1]<-5000

daily[2,1]<-5100





#reshape the data for pairwise comparison

library(reshape2)

daily_long<-melt(daily, id=)



#conduct simple test

wilcox.test(daily$pre,daily$post, paired=T) #produces desired results



#do the test again but now in a pairwise, which produces the same p-value
as in the simple test above

pairwise.wilcox.test(daily_long$value,daily_long$variable, paired=T)



#But now I the issues arise when testing more than two vectors.



#take three vectors this time

daily<-data.frame(pre=daily.intake, post=daily.intake, posttwo=daily.intake)



#add some differences

daily[1,1]<-5000

daily[2,1]<-5100

daily[10,3]<-9000

daily[11,3]<-9100



#the wilcox.test() produces a set of p-values

wilcox.test(daily$pre,daily$posttwo, paired=T)

 wilcox.test(daily$pre,daily$post, paired=T)

wilcox.test(daily$post,daily$posttwo, paired=T)



#and the pairwise.wilcox.test produces another set

pairwise.wilcox.test(daily_long$value,daily_long$variable, paired=T)







##And from the manual for pairwise.wilcox we get similar issues



#produces a given set of p-values

attach(airquality)

Month <- factor(Month, labels = month.abb[5:9])

## These give warnings because of ties :

pairwise.wilcox.test(Ozone, Month)

pairwise.wilcox.test(Ozone, Month, p.adj = "bonf")

detach()



#but if we want to test the rank difference between the 6th and 7th month
we get a p-value of 0.5775



testar skillnaden mellan 6e och 7e månaden – observ however that this data
is not paired which makes it different to the example I gave above.

#p-värdet är 0.5775



air<-subset(airquality, airquality$Month < 7)



#p-value is now 0.1925

wilcox.test(air$Ozone~air$Month)





What am I doing wrong here?

Best

Adel

-- 
Adel Daoud, PhD
Visiting researcher (post-doc)

Max Planck Institute for the Study of Societies / Max-Planck-Institut für
Gesellschaftsforschung
Paulstr. 3 | 506 76 Köln | Germany
Tel.: + 49 (0) 221 2767-534
daoud@mpifg.de



Department of Sociology and Work Science,  University of Gothenburg
Box 720
405 30 Göteborg, Sweden
Visiting address: Sprängkullsgatan 25, room K109
+46 031-786 41 73
Adel.daoud@sociology.gu.se




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