Hi Cedric, Thanks a lot for your help, after calculating U value using the formula from wikipedia I also found that the W given by R was in fact the U value that I could directly compared to table of critical value. Your advice were really good and useful. I would also be careful with the conclusions of the test due to the limitations you suggested! Have a nice day, Chloé
> > > Hi Chloe, > > first of all, I want to note, that you should be careful using the > WMW-test. > Even though it is often reported to be some sort of a "swiss-army-knife" > for > comparing two distributions, recent research on this test has revelaed > that it > is crucial what hypotheses you consider. Also the assumptions imposed to > the > test are critical. For the assumptions, the test basically is a test on > identical distributions. For your sample sizes, this is in my opinion > quite > problematic, as you can not really know what the population distributions > look > like. Furthermore, the test has shown to be quite strongly influenced by > differing variances in the two groups. All this is more or less valid for > not > necessarily small sample sizes, therefore I am not sure how much it might > affect your results. Therefore, caution should be adressed to the > interpretation of the results. > > On Friday, 20. August 2010 19:41:55 Chloe wrote: >> 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. > > Im a little bit puzzled on this one... I would agree with you. I can't > really > help you with this one right now, but doing the calculation of U manually > is > not really hard for your problem. All the values from method 2 are higher > than > the ones from method 1. So the ranking would be: > > method1 : 1,2,3 > method2: 4,5,6 > => W(rank sum)_m,n = 1 + 2 + 3 = 6 > > If I use the definition of U from > http://de.wikipedia.org/wiki/Mann-Whitney-U- > Test > I would calculate U = 0 , which goes with your formula: U = W - 6 = 6 - 6 > = 0, > what makes sense because the values of X are never greater than the ones > of Y. > (s. link: the formula for U with the two summations ) > > Thinking of that, the usage of W in R might simply be misleading and it > could > indeed represent U. > >> >> 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 > > For your sample sizes you can nicely use the critical value tables that > can be > found easily on the net. > > I hope I could help with your problem, if not, please feel free to ask > further. > > Best, > > Cedric > > ______________________________________________ > 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. > > > ______________________________________ > View message @ > http://r.789695.n4.nabble.com/U-value-from-wilcox-test-tp2332811p2332857.html > > To unsubscribe from U value from wilcox.test, click > http://r.789695.n4.nabble.com/template/NodeServlet.jtp?tpl=unsubscribe_by_code&node=2332811&code=Y2hsb2UuYm9ubmluZWF1QHVkZy5lZHV8MjMzMjgxMXwxNTg0NzMxMjA2 > -- View this message in context: http://r.789695.n4.nabble.com/U-value-from-wilcox-test-tp2332811p2333515.html Sent from the R help mailing list archive at Nabble.com. [[alternative HTML version deleted]]
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