May be an approach on parameter estimation (as effect size) is most interesting than a P value.
Best, Manuel 2014-03-24 8:39 GMT-06:00 Zoltan Botta-Dukat < botta-dukat.zol...@okologia.mta.hu>: > Dear All, > > Jari probably refer to that Mann-Whitney test suppose same shape of the > distributions (their positions may differ if H0 is false). > > Best wishes > Zoltan > > > 2014.03.24. 12:42 keltezéssel, Jari Oksanen írta: > > Except that t-test does not assume that *observations* are normally >> distributed, nor that variances are equal. >> >> Avoid non-parametric tests: they assume too much of data properties. >> >> For var.equal assumption in t.test, see ?t.test. >> >> Cheers, Jari Oksanen >> ________________________________________ >> From: r-sig-ecology-boun...@r-project.org [r-sig-ecology-bounces@r- >> project.org] on behalf of Richard Boyce [boy...@nku.edu] >> Sent: 24 March 2014 13:23 >> To: r-sig-ecology@r-project.org >> Subject: Re: [R-sig-eco] report out by t.test >> >> Mike, >> >> There is no way that your data meet the assumptions of a t-test (normal >> distributions, equal variance). A nonparametric Mann-Whitney (aka Wilcoxon) >> test is much better suited to your data. >> >> Here's what I got when I ran it: >> >> Q<-c(13,0,10,2,0,0,1,0,0,1,5) >> WD<-c(0,0,1,0,0,0,0,0,0,0,1) >> wilcox.test(Q,WD) >> >> Wilcoxon rank sum test with continuity correction >> >> data: Q and WD >> W = 86.5, p-value = 0.05119 >> alternative hypothesis: true location shift is not equal to 0 >> >> Warning message: >> In wilcox.test.default(Q, WD) : cannot compute exact p-value with ties >> >> This has a p-value quite close to 0.05, giving some evidence that there's >> a difference between your groups. Note that this you have different null >> and alternative hypothesis: groups are the same vs. groups are different. >> >> Rick Boyce >> >> On Mar 24, 2014, at 7:00 AM, r-sig-ecology-requ...@r-project.org<mailto: >> r-sig-ecology-requ...@r-project.org> wrote: >> >> Message: 1 >> Date: Sun, 23 Mar 2014 14:21:41 -0700 >> From: Michael Marsh <sw...@blarg.net<mailto:sw...@blarg.net>> >> To: r-sig-ecology@r-project.org<mailto:r-sig-ecology@r-project.org> >> Subject: [R-sig-eco] report out by t.test >> Message-ID: <532f5065.7030...@blarg.net<mailto:532f5065.7030...@blarg.net >> >> >> Content-Type: text/plain; charset=ISO-8859-1; format=flowed >> >> I test differences between frequency of hits of exotic annual forbs in >> plots on two sites, Q and WD. >> >> Q<-c(13,0,10,2,0,0,1,0,0,1,5) >> WD<-c(0,0,1,0,0,0,0,0,0,0,1) >> t.test(Q,WD) >> >> Welch Two Sample t-test >> >> data: Q and WD >> t = 1.9807, df = 10.158, p-value = 0.07533 >> alternative hypothesis: true difference in means is not equal to 0 >> 95 percent confidence interval: >> -0.3342006 5.7887460 >> sample estimates: >> mean of x mean of y >> 2.9090909 0.1818182 >> >> The p-value is greater than 0.05, thus does not reach the 95% confidence >> level, yet the difference in means is reported as not equal to 0. >> Am I encountering a one-sided versus two sided comparison that I don't >> understand, or is ther another explanation? >> >> Mike Marsh >> >> >> >> >> ================================ >> Richard L. Boyce, Ph.D. >> Director, Environmental Science Program >> Professor >> Department of Biological Sciences, SC 150 >> Northern Kentucky University >> Nunn Drive >> Highland Heights, KY 41099 USA >> >> 859-572-1407 (tel.) >> 859-572-5639 (fax) >> boy...@nku.edu<mailto:boy...@nku.edu> >> http://www.nku.edu/~boycer/ >> ================================= >> >> "One of the advantages of being disorderly is that one is constantly >> making exciting discoveries." - A.A. Milne >> >> >> >> >> >> >> [[alternative HTML version deleted]] >> >> _______________________________________________ >> R-sig-ecology mailing list >> R-sig-ecology@r-project.org >> https://stat.ethz.ch/mailman/listinfo/r-sig-ecology >> >> _______________________________________________ >> R-sig-ecology mailing list >> R-sig-ecology@r-project.org >> https://stat.ethz.ch/mailman/listinfo/r-sig-ecology >> > > > -- > Botta-Dukát Zoltán > -------------------------------- > Ökológiai és Botanikai Intézet > Magyar Tudományos Akadémia > Ökológiai Kutatóközpont > -------------------------------- > 2163. Vácrátót, Alkotmány u. 2-4. > tel: +36 28 360122/157 > fax: +36 28 360110 > botta-dukat.zol...@okologia.mta.hu > www.okologia.mta.hu > > > Zoltán BOTTA-Dukát > -------------------------------- > Institute of Ecology and Botany > Hungarian Academy of Sciences > Centre for Ecological Research > -------------------------------- > H-2163 Vácrátót, Alkomány u. 2-4. > HUNGARY > Phone: +36 28 360122/157 > Fax..: +36 28 360110 > botta-dukat.zol...@okologia.mta.hu > www.okologia.mta.hu > > > _______________________________________________ > R-sig-ecology mailing list > R-sig-ecology@r-project.org > https://stat.ethz.ch/mailman/listinfo/r-sig-ecology > -- *Manuel Spínola, Ph.D.* Instituto Internacional en Conservación y Manejo de Vida Silvestre Universidad Nacional Apartado 1350-3000 Heredia COSTA RICA mspin...@una.ac.cr mspinol...@gmail.com Teléfono: (506) 2277-3598 Fax: (506) 2237-7036 Personal website: Lobito de río <https://sites.google.com/site/lobitoderio/> Institutional website: ICOMVIS <http://www.icomvis.una.ac.cr/> [[alternative HTML version deleted]]
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