Hi Gian,

In an ANOSIM the value of the R statistic is at least as important as 
its statistical significance. So you must always look at both. Besides, 
if you have a lot of replicates you increase the chance of a small value 
of R being significantly different from zero.
I would say that 0.15 is a rather small value for R but one must always 
look at the data. In your particular case, with an R = 0.27, I would 
take a look to the nMDS of the data (using the same similarity measure 
as for ANOSIM, of course!). If it has a low stress, it gives a reliable 
picture of the positioning of your samples and if your samples are 
definitely grouped, you should also be able to see it in the nMDS. More 
important than any significance level is what you think your data is 
telling you!

Kind regards,

Dulce

Maria Dulce Subida

 

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Instituto de Ciencias Marinas de Andalucía (ICMAN)

Consejo Superior de Investigaciones Científicas (CSIC)

Campus Universitário Río San Pedro

11510 Puerto Real - Cádiz. España.

 

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Gian Maria Niccolò Benucci escribió:
> Barbara,
>
> Thank you very much, indeed I red the two values on a paper, that were of
> R=0.15 and P=0.006, and they states that the difference between the two
> groups is significant with them. So, my values, that are aloso higher, are
> better for sure to support the difference between my two community. Don't
> you agree?
> Thank you really much for all,
>
> G.
>
>
>
>
> 2010/6/8 <Barbara.Muhling@noaa.gov>
>
>   
>> Hi Gian,
>> I'm not sure if anyone else has answered your question yet.
>> But basically: the R statistic is a measure of the strength of the
>> difference between your 2 groups. It considers the variance both within,
>> and between, your groups. It can be from 0-1, with 1 the strongest
>> difference.
>> The significance value tests the significance of the difference, like
>> any other p-value. The statistics you derived from your ANOSIM analysis
>> are somewhat analogous to an F-statistic, and a p-value, for a
>> univariate ANOVA test.
>> When including the results in a paper or dissertation, it would be
>> normal to quote both values (R and significance).
>> I hope that's helpful,
>>
>> Barbara
>>
>>
>>
>> ----- Original Message -----
>> From: Gian Maria Niccolò Benucci <gian.benucci@gmail.com>
>> Date: Tuesday, June 8, 2010 4:57 am
>> Subject: [R-sig-eco] ANOSIM test
>>
>>     
>>> Hi everybody,
>>>
>>> I am tring to get some statistical evidence of differences between
>>> my two
>>> fungal mycorrhizal communities (samples from two different Host
>>> species)with ANOSIM test, that's my code:
>>>
>>>       
>>>> dist.ABCD <- vegdist(sqrt.ABCD.1)
>>>> attach(env.table1)
>>>> ABCD.ano <- anosim(dist.ABCD, Host)
>>>> summary(ABCD.ano)
>>>>         
>>> Call:
>>> anosim(dat = dist.ABCD, grouping = Host)
>>> Dissimilarity: bray
>>>
>>> ANOSIM statistic R: 0.272
>>>      Significance: 0.001
>>>
>>> Based on  999  permutations
>>>
>>> Empirical upper confidence limits of R:
>>>   90%    95%  97.5%    99%
>>> 0.0407 0.0554 0.0724 0.0952
>>>
>>> As I read in a paper, also an R value of 0.27 could be considered
>>> significant. So similar samples are inside the communities and not
>>> outsidethem, is it roght?
>>> Thank you for you time and patience,
>>>
>>>
>>> Gian
>>>
>>>
>>>
>>> --
>>> Gian Maria Niccolò Benucci
>>> Ph.D. Candidate
>>> Department of Applied Biology - University of Perugia
>>> Borgo XX Giugno, 74
>>> I-06121 - Perugia, ITALY
>>> Tel: +39.0755856433
>>> Fax: +39.0755856069
>>> Email: gian.benucci@gmail.com
>>>
>>>
>>> ----- Do not print this e-mail If you do not need it. Save paper
>>> and protect
>>> the environment! -----
>>>
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>>>
>>>
>>>       
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