[R] problem with wilcox.test()

[Ivan Calandra]

Dear useRs,

I am running a wilcox.test() on two subsets of a dataset and get exactly 
the same results although the raw data are different in the subsets.

mydata <- structure(list(cat1 = structure(c(2L, 2L, 2L, 2L, 2L, 2L, 1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L), .Label = c("high", "low"), 
class = "factor"), cat2 = structure(c(1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 
1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L), .Label = c("large", "small"), 
class = "factor"), var1 = c(2.012743, 1.51272, 1.328453, 1.2609935, 
1.617757, 1.8175455, 1.890035, 2.3652205, 1.295888, 1.5985145, 1.081813, 
1.856733, 2.366358, 2.27421, 1.727023, 2.230433, 5.272843, 3.7626355), 
var2 = c(0.00196, 0.0066545, 0.006188, 0.0058985, 0.004453, 0.005468, 
0.003773, 0.004742, 0.007525, 0.0081235, 0.004611, 0.0050475, 0.006643, 
0.0097335, 0.009213, 0.0049525, 0.006243, 0.006021)), .Names = c("cat1", 
"cat2", "var1", "var2"), row.names = c(NA, 18L), class = "data.frame")

#p-values are identical but W different for the first variable
wilcox.test(var1~cat1, data=mydata[mydata$cat2=="large",])
wilcox.test(var1~cat1, data=mydata[mydata$cat2=="small",])

#both p-values and W are identical for the second variable
wilcox.test(var2~cat1, data=mydata[mydata$cat2=="large",])
wilcox.test(var2~cat1, data=mydata[mydata$cat2=="small",])

Did I do something wrong or does it just have something to do with my 
dataset? Or is it just a coincidence?

Thank you in advance for your help,
Ivan

-- 
Ivan Calandra, ATER
University of Reims Champagne-Ardenne
GEGENAA - EA 3795
CREA - 2 esplanade Roland Garros
51100 Reims, France
+33(0)3 26 77 36 89
ivan.calandra at univ-reims.fr
https://www.researchgate.net/profile/Ivan_Calandra