[R-sig-ME] Is it worth testing for overdispersion?

Paul Johnson pauljohn32 at gmail.com
Thu Jun 23 20:00:46 CEST 2011

On Tue, Jun 21, 2011 at 6:53 AM, Iker Vaquero Alba <karraspito at yahoo.es> wrote:
>    Dear list:
>    I read some time ago in one of the posts that the option of implementing quasi- families in lmer had been removed. Is that right?

You are confusing 2 things.  Overdispersion can occur, it may be a problem.

But, we believe the "quasi" sorts of models are just wrong, and if you
are going to treat over dispersion, you need to treat it properly,
either by changing to a negative binomial model or by introducing a
"real" mixed effect, not a quasi approximation of a mixed effect.

Overdispersion would naturally be a property of a non-mixed model,and
you can test for it in many ways. Could I suggest to you Simon
Jackman's package, "pscl", which includes some nice tools for that.


Paul E. Johnson
Professor, Political Science
1541 Lilac Lane, Room 504
University of Kansas

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