R help - Jun 2005 - glm

Hi, list,

I have got a dataset on soil and plant. I would like to fit a "glm" to
my
data. My response variable is percentage data. That is percent of plant root
length colonized by Arbuscular micorrhiza fungi. Because of the nature of my
data, I am not quite sure whether gamma or gaussian distribution is suitable
for this type of data. If I use gamma distribution, which link function is
appropriate?

I appreciate your suggestions very much.

Thanks in advance!


Mahdi


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Mahdi Osman (PhD)
E-mail: m_osm at gmx.net
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I would not use gamma or gaussian for a proportion.  If the 
percentages were success or failure rate out of a known number of 
trials, then I'd try function "glm" with family = binomial or 
quasibinomial.  However, "percent of plant root length colonized" does
not sound to me like "glm" would be appropriate.  The beta
distribution
seems to me to be the most plausibly distribution to use.

	  I therefore did a search for "beta regression using 
"www.r-project.org" -> search -> "R site search". 
This revealed a
package "betareg" available from CRAN.  Have you considered that?  If 
no, I think I'd try it.

	  spencer graves

Mahdi Osman wrote:
> Hi, list,
> 
> I have got a dataset on soil and plant. I would like to fit a
"glm" to my
> data. My response variable is percentage data. That is percent of plant
root
> length colonized by Arbuscular micorrhiza fungi. Because of the nature of
my
> data, I am not quite sure whether gamma or gaussian distribution is
suitable
> for this type of data. If I use gamma distribution, which link function is
> appropriate?
> 
> I appreciate your suggestions very much.
> 
> Thanks in advance!
> 
> 
> Mahdi
> 
>