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
Sent from the R help mailing list archive at Nabble.com.
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
