Dear Tania,
Shouldn't that be varIdent(form = ~species.group) instead of
varIdent(form = ~1|species.group)?
Notice that this is untested as you did not provide a self-contained
example (as de posting guide asked you to do).
HTH,
Thierry
PS Next time try the mixed-models mailing list with questions about
nlme.
------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium
tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be
www.inbo.be
To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher
The plural of anecdote is not data.
~ Roger Brinner
The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey
-----Oorspronkelijk bericht-----
Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
Namens Tania BrenesArguedas
Verzonden: dinsdag 24 februari 2009 16:23
Aan: r-help at r-project.org
Onderwerp: [R] Initialize varFunc in R
Hi,
I am running R2.8.1 under Linux, and I am having trouble using the
variance functions in nlme
My basic model was something like:
model0 <- lme( log(growth) ~ light * species.group , data=data,
random=~light|species ) # with 20 odd species divided in 2 groups
Following the methods in Pinheiro&Bates I tried to put a variance
function in the model:
model1 <- update(model0, weights=varIdent(form = ~1|species.group) )
I got no error message or note but when I printed model1 the output
did not print any variance structure. It just didn't add any weights
or variance structure and model1 = model0
If instead I ran the varFunc sepparately:
varf1 <- varIdent(form = ~1|species.group)
varf.init <- Initialize(varf1, data)
model1 <- update(model0, weights=varf.init )
it works just fine, and I get an output with "Variance function:
Structure: Different standard deviations per stratum" and a change in
the diagnostic plots. So, this one works
Digging a little bit I noticed that the "initialize" in
Pinheiro&Bates
is not capitalized. And "initialize" in R is used by some other
package, such that each time I would try to follow the book I would
get an error message:
" c("\"varIdent\" is not a defined class",
"\"varFunc\" is not a
defined class")"
When I figured out that in R I had to use "Initialize" with uppercase,
I was able to make the above work.
Is this an bug with the R varFunc or am I doing something wrong?
Tania
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