[R] Proper syntax for using varConstPower in nlme
Michael A. Gilchrist
mikeg at utk.edu
Fri Oct 16 23:56:13 CEST 2009
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?
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
> Always do a plot of the result; the default plot gives you residual, and
> some indication how to proceed.
> View this message in context: http://www.nabble.com/Proper-syntax-for-using-varConstPower-in-nlme-tp25914277p25927578.html
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