[R] nlme iteration process, few questions

[Ken Beath]

> Recently I started using nlme intensively, and since it is all new for
> me, I have some questions. I am running nlme with
> control=list(verbose=TRUE) and during one lengthy fitting, I started
> watching the output for some clues, how to speed up the process. I
> noticed that in one case, the iteration process is alternating between
> two solutions. Here is an output of 5 iterations, after which I
> stopped the calculation:
>

I have sometimes fixed this problem by increasing the number of nlm  
and PNLS iterations. It can be due to the random effects estimates  
being too small, so not necessary in the model, or I suspect that a  
high correlation between the random effects would also produce the  
problem.

The usual method is to try fitting simpler models by removing random  
effects or with structured covariance matrix and then use the  
estimates of the fixed effects as starting values for more complex  
models.

HTH
Ken