[R-sig-ME] Is there a way to deal with errors such as this?

[Rolf Turner]

On 16/12/19 10:08 am, D. Rizopoulos wrote:

<SNIP>

> Moreover, if the main aim of the simulation is to assess some properties 
> of the model and not of the optimization algorithm, you could help the 
> optimization procedure by supplying as starting values for the model 
> parameters the true parameters values from which you simulate the data.

That is a very appealing idea, but I'm afraid that I find the help for 
glmer() to be utterly opaque in terms of the "start" argument.  May I 
impose upon you for some help to get me, uh, started? ( :-) )

Explicitly, suppose that I have a data frame "protoX", and I fit a model:

protoFit <- glmer(cbind(Dead,Alive) ~ (0+Trt)/Dose + (Dose | Rep),
              data=protoX,family=binomial(link="probit"))

Suppose that I then simulate data from protoFit:

X <- protoX
X[,c("Dead","Alive") <- simulate(protoFit)[,1]

The "true" values of the parameters in respect of fitting a model to X, 
will be the fitted parameters contained in protoFit.  How do I specify 
these as starting values in a call to glmer()?

I would do something like:

fit <- glmer(cbind(Dead,Alive) ~ (0+Trt)/Dose + (Dose | Rep),
              data=X,family=binomial(link="probit"),start=???)

What do I use for "???" ?  The help seems to indicate that it should be 
a list with components "fixef" and "theta".  I would conjecture that I'd 
get the "fixef" component as fixef(protoFit).  However it is totally 
mysterious to me how I would get "theta" (or even what "theta" *is*).

Can you (or someone) point me in the right direction?  (An *example* of 
the use of "start" in the help file would have been nice. :-( )

cheers,

Rolf

-- 
Honorary Research Fellow
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276