[R-sig-eco] report out by t.test

[Zoltan Botta-Dukat]

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-bounces at r-project.org [r-sig-ecology-bounces at r-project.org] on behalf of Richard Boyce [boycer at nku.edu]
> Sent: 24 March 2014 13:23
> To: r-sig-ecology at 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-request at r-project.org<mailto:r-sig-ecology-request at r-project.org> wrote:
>
> Message: 1
> Date: Sun, 23 Mar 2014 14:21:41 -0700
> From: Michael Marsh <swamp at blarg.net<mailto:swamp at blarg.net>>
> To: r-sig-ecology at r-project.org<mailto:r-sig-ecology at r-project.org>
> Subject: [R-sig-eco] report out by t.test
> Message-ID: <532F5065.7030102 at blarg.net<mailto:532F5065.7030102 at 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)
> boycer at nku.edu<mailto:boycer at 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 at r-project.org
> https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
>
> _______________________________________________
> R-sig-ecology mailing list
> R-sig-ecology at 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.zoltan at 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.zoltan at okologia.mta.hu
www.okologia.mta.hu