R-sig-ecology Digest, (80/1): Binomial GLM: Is it the right way?
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 -- View this message in context: http://r-sig-ecology.471788.n2.nabble.com/Re-R-sig-ecology-Digest-80-1-Binomial-GLM-Is-it-the-right-way-tp7579191p7579192.html Sent from the r-sig-ecology mailing list archive at Nabble.com.
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