[R-sig-ME] glmmadmb beta - error

[Highland Statistics Ltd]

The fact that the software runs properly, or not, is irrelevant. You 
need to have at least 10 - 15 - 20 observations per regression parameter 
for an ordinary regression model. Using only 50 observations for the 
beta mixed model is not the best thing to do. Maybe this is not what you 
wanted to hear.

Alain

On 23/02/2016 12:43, Ludovico Frate wrote:
> Thanks for your reply!
> I successfully fitted another model with the same number of 
> observation (and structure) and the only difference is that cover 
> values were more centered on the mean values.
>
> I just tried to fit the same model by adding a constant cover value to 
> all observations (+0.20) and the model works
>
> I think that the issue derives from very small cover values...
>
> summary(cover_lf$cover) (divided by 100)
>
> Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
> 0.0000  0.0050  0.0130  0.0252  0.0385  0.1300
>
> Call:
> glmmadmb(formula = cover ~ time + summit + direction + (1 | plot_id),
>     data = cover_lf, family = "beta")
>
> AIC: -228.7
>
> Coefficients:
>             Estimate Std. Error z value Pr(>|z|)
> (Intercept)  -1.2241     0.0476  -25.72   <2e-16 ***
> time2015     -0.0688     0.0337   -2.04   0.0410 *
> summitMAC    -0.1407     0.0437   -3.22   0.0013 **
> directionN    0.1028     0.0574    1.79   0.0733 .
> directionS    0.0760     0.0624    1.22   0.2238
> directionW    0.0986     0.0574    1.72   0.0860 .
> ---
> Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
>
> Number of observations: total=50, plot_id=25
> Random effect variance(s):
> Group=plot_id
>             Variance  StdDev
> (Intercept) 0.003394 0.05826
>
> Beta dispersion parameter: 403.43 (std. err.: 0.00012419)
>
> Log-likelihood: 122.34

-- 
Dr. Alain F. Zuur

First author of:
1. Beginner's Guide to GAMM with R (2014).
2. Beginner's Guide to GLM and GLMM with R (2013).
3. Beginner's Guide to GAM with R (2012).
4. Zero Inflated Models and GLMM with R (2012).
5. A Beginner's Guide to R (2009).
6. Mixed effects models and extensions in ecology with R (2009).
7. Analysing Ecological Data (2007).

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