[R] U value from wilcox.test

[Chloe]

Dear all, 
I want to compare the efficiency of 2 methods in extracting proteins from
algal samples. I collected 6 independant algal samples and I extracted 3 by
the method 1 and 3 others by the method 2.
So I have 2 groups of 3 samples, that are not paired. I would like to know
if the results obtained by these 2 methods are significantly different, I
hope method 2 to be more efficient than method 1. As I have few data I went
for the Mann-whitney test:

method1=c(35,40,56)
method2=c(90,110,115)
wilcox.test(method1,method2,paired=FALSE,alternative="less")

  Wilcoxon rank sum test

data:  method1 and method2 
W = 0, p-value = 0.05
alternative hypothesis: true location shift is less than 0 

As I have a small number of samples, I would prefer to compare the U value
of the Mann-Whitney test to critical value for table rather than to rely on
the p-value.

Is W value correspond to U value ?
>From the help I understand that W=U+m*(m+1)/2, is this true ?
In the case it is, my U values would be U=W-6=-6!! I thought that a U value
could not be neagtive.

I would be happy to have any information about how to obtain the U value
from the Mann-Withney test (wilcox.test()) in order to be able to compare it
with table of critical U value commonly found.
Thanks a lot for your time and help
Have a nice day,
Chloé


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