[R-sig-eco] interpreting adonis results

[gabriel singer]

... dangerous wording, there could in fact be a location effect of 
'location' and/or a dispersion effect of 'location'.

Gian, I suggest you add a test of a dispersion effect using the function 
betadisper(), then you know a bit more about the type of effect.

gabriel

On 11/16/11 11:02 PM, Gavin Simpson wrote:
> On Wed, 2011-11-16 at 03:43 +0100, Gian Maria Niccolò Benucci wrote:
>> Hi all,
>>
>> I had 84 samples collected in 7 different sites.
>> In each sample were individuated the different fungal species and recorded.
>> I would test if exist a real difference between the sites and if exist a
>> sort of site effect that structure the fungal communities...
>> Then, I did adonis test
>>
>>> adonis(community.sq ~ location, data=env.table, permutations=999)
>> Call:
>> adonis(formula = community.sq ~ location, data = env.table, permutations =
>> 999)
>>
>>            Df SumsOfSqs MeanSqs F.Model      R2 Pr(>F)
>> location   6    12.593 2.09886  6.8867 0.34922  0.001 ***
>> Residuals 77    23.467 0.30477         0.65078
>> Total     83    36.060                 1.00000
>> ---
>> Signif. codes:  0 *** 0.001 ** 0.01 * 0.05 . 0.1   1
>>
>>
>>
>> The significance is  R2=0.349 at P=0.001
>> Can I assure that exist a strong site effect in structuring the communities
>> in each site?
> Depends. The test is one of no effect of `location`. You have found
> evidence against this hypothesis and thus could reject this hypothesis,
> instead accepting the alternative hypothesis that there is an effect of
> `location`. As to the strength of this effect? ~35% of the sums of
> squares can be explained by `location`. Substantially more of the
> variance remains unexplained. As I know nothing about your subject area,
> I am unable to comment further on the strength of the relationship.
>
> Seeing as many ecologists whose work I read would say an effect is
> significant if the p-value was>= 0.05. Not that I subscribe to this way
> or working, but by that criterion, you have identified a significant
> `location` effect.
>
> HTH
>
> G
>
>> Thanks for helping,
>>
>> G.
>>
>> 	[[alternative HTML version deleted]]
>>
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-- 
Dr. Gabriel Singer
Department of Limnology - University of Vienna
and Wassercluster Lunz Biologische Station GmbH
+43-(0)664-1266747
gabriel.singer at univie.ac.at