[R] lme questions re: repeated measures & covariance structure

[Christoph Scherber]

Dear Mark,

I would include the repeated measure as the smallest stratum in the random effects specification:

random=~1|sampleunit/year

Setting up user-defined variance structures should be possible using for example:

weights=varPower(form=~habitat)

or also try out the available corStruct() classes (found in Pinheiro and Bates 2000)

HTH
Christoph




Mark Na schrieb:
> Hello,
> 
> We are attempting to use nlme to fit a linear mixed model to explain bird
> abundance as a function of habitat:
> 
> 
> lme(abundance~habitat-1,data=data,method="ML",random=~1|sampleunit)
> 
> The data consist of repeated counts of birds in sample units across multiple
> years, and we have two questions:
> 
> 1) Is it necessary (and, if so, how) to specify the repeated measure
> (years)? As written, the above code does not.
> 
> 2) How can we specify a Toeplitz heterogeneous covariance structure for this
> model? We have searched the help file for lme, and the R-help archives, but
> cannot find any pertinent information. If that's not possible, can we adapt
> an existing covariance structure, and if so how?
> 
> Thanks, Mark
> 
> 	[[alternative HTML version deleted]]
> 
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-- 
Dr. rer.nat. Christoph Scherber
University of Goettingen
DNPW, Agroecology
Waldweg 26
D-37073 Goettingen
Germany

phone +49 (0)551 39 8807
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Homepage http://www.gwdg.de/~cscherb1