[R] extracting dispersion parameter from quasipoisson lmer model

liujb liujulia7 at yahoo.com
Tue Mar 10 20:48:26 CET 2009


I have the exact same question: how to extract dispersion parameter from
lmer() quasipoisson model?

I did a search here and saw this post. However, I do not see answer. Could
somebody please help?

Thank you very much,

wayne hallstrom wrote:
> Hi,
>   I would like to obtain the dispersion parameter for a quasipoisson model
> for later use in calculating QAIC values for model comparison.Can anyone
> suggest a method of how to go about doing this?
>   The idea I have now is that I could use the residual deviance divided by
> the residual degrees of freedom to obtain the dispersion parameter. The
> residual deviance is available in the summary output for the lmer model.
> The problem with this idea is that the residual degrees of freedom is not
> directly available. I have been assuming that the estimated/approximate DF
> can be obtained through the formula listed below, and then subsequently
> used to obtain the dispersion parameter as described above. Is this
> correct?
>   samp.m4 <- mcmcsamp(m4, n = 1000)
> print(summary(samp.m4))
> (eDF <- mean(samp.m4[,"deviance"]) - deviance(m4, REML=FALSE)) 
>   # potentially useful approximate DF?
>   However, rather than going through this roundabout, there appears to be
> an easier way to obtain the dispersion parameter. I have noted that the
> 'quasibinomial' produces a dispersion parameter in the model output, but
> the 'quasipoisson' output does not contain this useful number. Given that
> each of my models is fit by lmer with a quasipoisson distribution, the
> program must be internally calculating the dispersion parameter as it
> runs. Perhaps there is a way to obtain this number directly since I assume
> it has already been calculated? Can somone who has experience with this
> code provide some advice on whether this is possible and how I might
> manage to do this?
>   Thank you for your time,
>   Wayne Hallstrom
>   403-370-3832
> ---------------------------------
> 	[[alternative HTML version deleted]]
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