[R] Proper syntax for using varConstPower in nlme

[Dieter Menne]

Michael A. Gilchrist wrote:
> 
> ---------------------------------------------
>>  nlme(Count ~ quad.PBMC.model(aL, aN, T0),
> +   data = tissueData,
> +   weights = varConstPower(form =~ Count),
> +   start = list( fixed = c(rep(1000, 8), -2, -2) ),
> +   fixed = list(T0 ~ TypeTissue-1, aL ~ 1, aN ~ 1),
> +   random = aL + aN ~ 1|Tissue
> +   )
> Error in MEestimate(nlmeSt, grpShrunk) :
>    Singularity in backsolve at level 0, block 1
>>
>>
> 

You could use varPower(form=~fitted()) (the default, also varPower()). In my
experience this runs into some limitation quickly with nlme, because some
boundary conditions make convergence fail.

Try varPower(fixed = 0.5) first and play with the number.

You should only use varConstPower when you have problems with values that
cover a large range, coming close to zero, which could make varPower go
havoc.

Always do a plot of the result; the default plot gives you residual, and
some indication how to proceed.

Dieter


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