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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