[R] MuMIn Random Effects Variance

[Stephen Jane]

Hello,

I am using a negative binomial distribution in glmmADMB to fit a mixed model 
and then using the MuMIn package to get model averaged coefficients.  As far as 
I can tell, this approach gives no estimates for the variance of the random 
effects.  I have been taking these from the top model according to AIC.  Is 
there a preferred approach to getting these?

I would be grateful for any insights.

Stephen Jane

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[R] step halving factor reduced below minimum

[Stephen Jane]

Hello,
 
I am attempting to use nlme to model the response of 19 groups.  I am able to 
get a reasonable fit of all of the groups to the observed data using an 
exponential decay model (a*exp-x*b).  The problem is that when I plot the 
fitted values to residuals, it demonstrates a pattern of increasing variance.
 
I attempt to model this variance using
 
Sig3.nlme - update(Sig2.nlme, weights = varConstPower(power=0.1),
   verbose = TRUE)
 
I get the message that 'step halving factor reduced below minimum in PNLS 
step'.  Some of the curves are actually flat lines where there is a value of 0 
for the response at every observation within the group.  My thought is that, 
because this corresponds to 0 variance, it is causing problems for the pnls.  
Don't know that this is the correct interpretation.
 
Is there anyway to model this type of data using nlme or do I need to find 
another option?
 
Thanks for any help.
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