[R] Initialize varFunc in R

[ONKELINX, Thierry]

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 

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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.
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-----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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