[R] lme: random effects of a quadratic term

[Prof Brian Ripley]

On Sat, 26 Mar 2005, Montse Rue wrote:

> I am estimating the following model:
>
> so2.lme<-lme(so2~1+I(alcadakm^2)+dia,data=subjectes2,na.action=na.omit)
>
> And when I try to plot the random effects of the quadratic term with
> respect to a covariate (mam) I get an error:
>
>> so2.lmeRE<-ranef(so2.lme,augFrame=T)
>> plot(so2.lmeRE,form=I(alcadakm^2)~mam)
> Error in plot.ranef.lme(so2.lmeRE, form = I(alcadakm^2) ~ mam ) :
> 	Only single effects allowed in left side of form.
>
> Any suggestion?

Try renaming the variable, e.g. alcadakm2 = alcadakm^2, and refit.

NB: this is not `a quadratic term' but a transformed variable, since you 
used I().

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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