[R-sig-eco] R-sig-ecology Digest, (80/1): Binomial GLM: Is it the right way?

[Abdoul Dia]

Dear Frate, I don't know if I do understand well your problem but 
Binomial suppose that you have a binary response variable. For 
proportions, you should use other models otherwise you should specifiy 
proportions of success vs failure.

Hope this could help.

Cheers
Abdoul

On 14-11-01 10:29 AM, Ludovico Frate wrote:
> Dear Class, thank you for your reply.
> Maybe I have not been very clear in the exposition. I'll try to reformulate.
>
> "the likelihood of colonizing or becoming extinct" is a proportion so the
> binomial distribution should be still ok!
>
> Considering only colonizing species, or arrivals, for a given plot. This
> means that a species is not new in overall sampled flora but only for that
> plot (i.e. *local* colonizer).
>
> PLOTID;TRAITS;NUMBER OF NEW COLONIZERS;NUMBER OF SUCCESS;PROPORTION
> 1;	ChF;	4;	0;	0
> 1;	ChC;	4;	0;	0
> 1;	ChR;	4;	0;	0
> 1;	ChSC;4;	0;	0
> 1;	ChS;	4;	1;	0,25
> 1;	Ph;	4;	0;	0
> 1;	Geo;	4;	0;	0
> 1;	HeB;	4;	0;	0
> 1;	HeC;	4;	0;	0
> 1;	HeR;	4;	0;	0
> 1;	HeS;	4;	3;	0,75
> 1;	Th;   4;	0;	0
> 2;	ChF;	17;	0;	0
> 2;	ChC;	17;	0;	0
> 2;	ChR;	17;	1;	0,058823529
> 2;	ChSC;17;	0;	0
> 2;	ChS;	17;	2;	0,117647059
> 2;	Ph;	17;	0;	0
> 2;	Geo;	17;	1;	0,058823529
> 2;	HeB;	17;	0;	0
> 2;	HeC;	17;	0;	0
> 2;	HeR;	17;	2;	0,117647059
> 2;	HeS;	17;	7;	0,411764706
> 2;	Th;	17;	4;	0,235294118
> 3;	ChF;	16;	0;	0
> 3;	ChC;	16;	0;	0
> 3;	ChR;	16;	2;	0,125
> 3;	ChSC;16;	0;	0
> 3;	ChS;	16;	0;	0
> 3;	Ph;	16;	0;	0
> 3;	Geo;	16;	2;	0,125
> 3;	HeB;	16;	0;	0
> 3;	HeC;	16;	0;	0
> 3;	HeR;	16;	1;	0,0625
> 3;	HeS;	16;	9;	0,5625
> 3;	Th;	16;	2;	0,125
> .       .       .       .
> .       .       .       .
> .       .       .       .
> 24    .       .       .
>
>
> for example, in the table, the plot 1 has 4 new species found of which 1 is
> a ChS and 3 are HeS with a proportion of 0.25 and 0.75 respectively.
>
> In a GLM model, the response variable should be PROPORTION (that I have
> improperly called PROBABILITY)
>
> An hypothetical model could be:
> M0<-glm(PROPORTION~TRAITS, family = binomial, weights = NUMBER OF NEW
> COLONIZERS, data = ...)
>
>
> Does it make sense?
>
>
> For the second question, I agree with you that "the probability of a species
> colonizing a plot not only depends on which life form that particular
> species has".
>
> Regards,
> Ludovico
>
>
>
> --
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