[R-sig-ME] random effect variance per treatment group in lmer

[Andrew Robinson]

Hi David,

as far as I am aware, there is no option for stratifying the variance
of random effects in either lme or lmer.  One can stratify the
variance of the innermost residuals in lme, but that is different than
what you are asking for. 

Cheers,

Andrew


On Tue, Jul 10, 2007 at 10:23:21AM -0400, Afshartous, David wrote:
> 
> All,
> I didn't receive a response to the query below sent to the general
> R-help mailing list so figured I'd try this mailing list.  Apologies
> in advance if this is an overly simplistic question for this list; I
> just started 
> w/ lmer after not using lme for awhile.
> Cheers,
> Dave 
> 
> 
> 
> 
> ___________________________________________________________
> 
> All,
>  
> How does one specify a model in lmer such that say the random effect for
> 
> the intercept has a different variance per treatment group?  
> Thus, in the model equation, we'd have say b_ij represent the random
> effect
> for patient j in treatment group i, with variance depending on i, i.e,
> var(b_ij) = tau_i.
>  
> Didn't see this in the docs or Pinherio & Bates (section 5.2 is specific
> for 
> modelling within group errors).  Sample repeated measures code below is
> for 
> a single random effect variance, where the random effect corresponds to
> patient.
> cheers,
> dave
>  
>  
> z <- rnorm(24, mean=0, sd=1)
> time <- factor(paste("Time-", rep(1:6, 4), sep="")) 
> Patient <- rep(1:4, each = 6) 
> drug <- factor(rep(c("D", "P"), each = 6, times = 2)) ## P = placebo, D
> = Drug
> dat.new <- data.frame(time, drug, z, Patient) 
> fm =  lmer(z ~ drug + time + (1 | Patient), data = dat.new )
> 
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-- 
Andrew Robinson  
Department of Mathematics and Statistics            Tel: +61-3-8344-9763
University of Melbourne, VIC 3010 Australia         Fax: +61-3-8344-4599
http://www.ms.unimelb.edu.au/~andrewpr
http://blogs.mbs.edu/fishing-in-the-bay/