What types of models are you running?


There may be two issues at play.


1. Likelihood ratio tests are only for nested models (i.e. models where the
variables in one model are a subset of the variables in the other) so by
definition there will always be a difference in degrees of freedom.


2. With mixed models you can only use a likelihood ratio test when the model
returns a deviance score - so not for generalized linear mixed models in
most cases (though I believe that you can use LRT's if they are estimated
using a type of numerical integration, but not any sort of quasi-likelihood
like PQL)


Andrew

On Fri, Jun 17, 2011 at 11:12 AM, Arnaud Mosnier <a.mosnier@gmail.com>wrote:

> Dear list,
>
> If I do not make a mistake, use of Likelihood ratio test is precluded
> when two models have the same number of degree of freedom.
> Is there a way to test which one is the best when both are close in
> AIC value (difference < 5) or do I have to conclude that they are
> "equivalent" ?
>
> Thanks
>
> Arnaud
>
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