R help - Feb 2005 - testing equality of variances across groups in lme?

Hello. I am fitting a two-level mixed model which assumes equality of
variance in the lowest-level residuals across groups.  The call is:

 

fit3<-lme(CLnNAR~CLnRGR,data=meta.analysis,

+ na.action="na.omit",random=~1+CLnRGR|study.code)

 

I want to test the assumption of equality of variances across groups at
the lowest level.  Can someone tell me how to do this?  I know that one
can test a nested model in which the residuals are given weights, such
as:

 

fit3<-lme(CLnNAR~CLnRGR,data=meta.analysis,

+ na.action="na.omit",random=~1+CLnRGR|study.code,
weights=varPower(form~CLnRGR))

 

This allows the residual variances to increase with the independent
variable. However, I have no expectation of how the variances might
change, only that they might be different across groups. 

 

 

Bill Shipley

Subject Matter Editor, Ecology

North American Editor, Annals of Botany

Département de biologie, Université de Sherbrooke,

Sherbrooke (Québec) J1K 2R1 CANADA

Bill.Shipley@USherbrooke.ca

 <http://callisto.si.usherb.ca:8080/bshipley/>
http://callisto.si.usherb.ca:8080/bshipley/

 


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> I am fitting a two-level mixed model which assumes equality of
> variance in the lowest-level residuals across groups. 
> The call is:
> 
> fit3<-lme(CLnNAR~CLnRGR,data=meta.analysis,
> + na.action="na.omit",random=~1+CLnRGR|study.code)
I assume that CLnRGR is a factor and thus the groups which might have
different variance in residuals are given by the different levels of
CLnRGR!?
> ...
> know that one can test a nested model in which the residuals are
> given weights, such as:
You are on the right track here, just use:

fit4<-lme(CLnNAR~CLnRGR,data=meta.analysis, na.action="na.omit",
random=~CLnRGR|study.code, weights= varIdent (form= ~ 1 | CLnRGR))

with anova (fit3, fit4) you can test whether these weights improve the fits
statistically.

Lorenz
- 
Lorenz Gygax, Dr. sc. nat.
Centre for proper housing of ruminants and pigs
Swiss Federal Veterinary Office
agroscope FAT T?nikon, CH-8356 Ettenhausen / Switzerland