[R-sig-ME] syntax lmer random effects

[Doran, Harold]

Nathan:

Here are a few small examples given what you have provided below. To use lmer with an intercept only and random intercepts:

fit1 <-  lmer(z ~ 1 + (1|ID), data, method='ML')

Now, to fit a model with a fixed effect, 'x', and to allow for the intercept and for the variable x to vary randomly, use the following:

fit2 <-  lmer(z ~ x + (x|ID), data, method='ML')

Notice that the random terms are always (x|ID), where x is the variable you want to have as the random effect, followed by a "pipe" (|) and then the grouping variable. In this example the grouping variable is ID. So, you have a random variable x, "given", or "conditional on" ID.

Also notice I use method='ML'. You said you wanted to compare models and these models differ in their fixed effects. REML is the default estimation procedure, and so you need this portion of code. 

> -----Original Message-----
> From: r-sig-mixed-models-bounces at r-project.org 
> [mailto:r-sig-mixed-models-bounces at r-project.org] On Behalf 
> Of Nathan Leon Pace, MD, MStat
> Sent: Thursday, July 26, 2007 1:39 PM
> To: r-sig-mixed-models at r-project.org
> Subject: [R-sig-ME] syntax lmer random effects
> 
> Hi,
> 
> Even after reading the various help files, I don¹t really 
> understand the formatting of the grouping factor.
> 
> My model is z ~ x*y with 1 grouping factor. I wish to compare 
> models with only an intercept random effect, random effects 
> for intercept and x, random effects for intercept and y, 
> random effects for intercept plus x and y (independent and 
> not independent), and random effects for all fixed effects 
> (independent and not independent).
> 
> My data frame has 360 rows with 24 levels of the grouping factor
> 
> > sessionInfo()
> R version 2.5.1 (2007-06-27)
> i386-apple-darwin8.9.1
> 
> locale:
> en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
> 
> attached base packages:
> [1] "stats"     "graphics"  "grDevices" "utils"     
> "datasets"  "methods"
> "base"     
> 
> other attached packages:
>         lme4       Matrix      lattice
>  "0.99875-4" "0.999375-0"     "0.16-2"
> 
> Insights will be appreciated.
> 
> Nathan
> --
> Nathan Leon Pace, MD, MStat
> University of Utah
> n.l.pace at utah.edu
> W: 801.581.6393
> F: 801.581.4367
> M: 801.205.1019
> 
> 
> 
> 
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