Hi Fred,
For this models you can use the negative.binomial() function within the *
MASS* package for the Negative Binomial model, and the zeroinfl () function
of the *pscl *package for the Zero Inflated Poission models.

Hope you find it useful.

Antonio.

On Wed, May 18, 2011 at 5:54 PM, Pavel Goldstein <dao4free@gmail.com> wrote:

> Hello Fred,
> You can use Zerro Inflated Poisson or Negative binomial models.
>
> Pavel
>
> On Wed, May 18, 2011 at 6:46 PM, Fred Takahashi <fredtaka@gmail.com>
> wrote:
>
> > Hello, just a basic question: what distribution I should use to
> > analyze continuous data which can had zero or positive values (eg.
> > mass of grasses in plots assuming that zero mass is meaningful)? My
> > expected analysis is in the framework of GLM or GLMM (or
> > alternatively, GAM / GAMM).
> >
> > One idea I had is to add 0.0001 to all values and use family=Gamma.
> > That is a good approach?
> >
> > If a better choice is to use a different distribution, suggestions of
> > packages to do that are welcome.
> >
> > Thanks,
> > Fred Takahashi
> > Universidade de Brasília - Brasil
> >
> > _______________________________________________
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> > R-sig-ecology@r-project.org
> > https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
> >
>
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>


-- 
Antonio Jesús Pérez Luque
Grupo de Ecología Terrestre.
Departamento de Ecología.
Centro Andaluz de Medio Ambiente, Universidad de Granada - Junta de
Andalucía
Avda. del Mediterráneo S/N. 18006. Granada. España
Tel: +34 958241000 (ext. 31202)
Fax: +34 958137246
email: ajperez@ugr.es
blog: http://ajperezluque.blogspot.com/
http://observatoriosierranevada.iecolab.es
http://iecolab.es

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