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.zoltan@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-bounces@r-project.org [r-sig-ecology-bounces@r-
>> project.org] on behalf of Richard Boyce [boycer@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-request@r-project.org<mailto:
>> r-sig-ecology-request@r-project.org> wrote:
>>
>> Message: 1
>> Date: Sun, 23 Mar 2014 14:21:41 -0700
>> From: Michael Marsh <swamp@blarg.net<mailto:swamp@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: <532F5065.7030102@blarg.net<mailto:532F5065.7030102@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@nku.edu<mailto:boycer@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
>>
>>
>>
>>
>>
>>
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>>
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>
>
> --
> 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@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@okologia.mta.hu
> www.okologia.mta.hu
>
>
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