[R] Proper syntax for using varConstPower in nlme

[Michael A. Gilchrist]

Hi Dieter,

Thanks for the reply.  I had played with the initial conditions, but 
apparently not enough.  I finally found some that avoided the singularity 
issue.  Many thanks.

More generally, I went over the documentation yet again in P&B and I still 
find it a bit confusing.  They talk about using form = ~fitted(.) when 
discussing varPower, but the rest of the documentation seems to suggest that 
"form = ~..." should be used to indicate which covariate you assume the 
variance changes with.

Could you or someone else provide some clarification?

Thanks.

Mike

On Fri, 16 Oct 2009, Dieter Menne wrote:

>
>
>
> 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
>
>
> -- 
> View this message in context: http://www.nabble.com/Proper-syntax-for-using-varConstPower-in-nlme-tp25914277p25927578.html
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