Overdispersion parameter: size vs theta

[Charlotte Taelman]

Dear INLA community,

A zero-inflated negative binomial model renders me the following output of hyperparameters:

I want to sample from a negative binomial distribution, using mu from another dataset and size from the INLA hyperparameters.

I am however confused about the use of names between base-R and R-INLA: 

-  package rnbinom: "An alternative parametrization (often used in ecology) is by the mean mu (=μ, see above), and size, the dispersion parameter, where prob = size/(size+mu) 

- in the inla.doc: "output.name size for the nbinomial observations (1/overdispersion)"

is size = overdispersion or 1/overdispersion?

Kind regards and thank you in advance,

Charlotte

[Helpdesk (Haavard Rue)]

please check

inla.doc("^nbinomial")

which spell out how overdispersion represented.

run the examples in the end to verify

On Wed, 2025-12-10 at 09:00 -0800, Charlotte Taelman wrote:
> Dear INLA community,
>
> A zero-inflated negative binomial model renders me the following output of
> hyperparameters:

> Screenshot 2025-11-27 111539.png
>
> I want to sample from a negative binomial distribution, using mu from another
> dataset and size from the INLA hyperparameters.
> I am however confused about the use of names between base-R and R-INLA: 
>
> -  package rnbinom: "An alternative parametrization (often used in ecology) is
> by the mean mu (=μ, see above), and size, the dispersion parameter,
> where prob = size/(size+mu) 
> - in the inla.doc: "output.name size for the nbinomial observations
> (1/overdispersion)"
>
> is size = overdispersion or 1/overdispersion?
>
> Kind regards and thank you in advance,
> Charlotte
>
>
>
>

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--
Håvard Rue
he...@r-inla.org

[Charlotte Taelman]

Thank you, I have figured it out!

Op do 11 dec 2025 om 08:06 schreef Helpdesk (Haavard Rue) <he...@r-inla.org>: