Sorry that I did not answer you earlier, I would like to thank you again for your advice and the time you dedicated to my problem. In my case it was not possible to increase the sample size but I would take your advice into account for the next sampling and also for presenting these results! Thanks again, have a nice day! Best wishes, Chloé
Le 21/08/2010 16:18, Cedric Laczny [via R] a écrit : > Glad that I could help :) > Another thing that came to my mind ist that when you simply look at > the values > of the different groups, they differ quite strongly in my opinion. > They are > between two and three times higher in the second group than the ones > from the > first group. Therefore it would be a good idea to increase your sample > sizes > and see if that trend can be observed further. I know that this is not > always > feasible. > IMHO statistics are nice and can help to gain insight into various > things. > However, for small sample sizes, talking about "statistical > significance" is in > my opinion always a bit tricky. > If I had to present such results, I would rather refer to the folds by > which > the data differ, rather than talking about statistics. Because > "statistical > significance" and "relevance" are two different "measures" ;) > > Best, > > Cedric > > On Saturday, 21. August 2010 15:58:50 Chloe wrote: > > > 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, > > >> ChloÃÂé > > > > > > 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 > > > > > > ______________________________________________ > > > [hidden email] </user/SendEmail.jtp?type=node&node=2333531&i=0> > 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 > <http://r.789695.n4.nabble.com/U-value-from-wilcox-test-tp2332811p2332857?by-user=t>. > > > > > html > > > > > > To unsubscribe from U value from wilcox.test, click > > > > http://r.789695.n4.nabble.com/template/NodeServlet.jtp?tpl=unsubscribe_by > <http://r.789695.n4.nabble.com/template/NodeServlet.jtp?tpl=unsubscribe_by&by-user=t> > > > > _code&node=2332811&code=Y2hsb2UuYm9ubmluZWF1QHVkZy5lZHV8MjMzMjgxMXwxNTg0N > > > zMxMjA2 > > ______________________________________________ > [hidden email] </user/SendEmail.jtp?type=node&node=2333531&i=1> > 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-tp2332811p2333531.html > > To unsubscribe from U value from wilcox.test, click here > <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-tp2332811p2529398.html Sent from the R help mailing list archive at Nabble.com. 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