[R] lme: error message with random=~1

[Thomas Petzoldt]

Dimitris Rizopoulos wrote:
> Hi Thomas,
> 
> "random=~1" works if your data frame is in "groupedData" format, check 
> this:
> 
> # Orthodont is in groupedData format
> fm1 <- lme(distance~age+Sex, data=Orthodont, random=~1)
> #####
> dat <- as.data.frame(Orthodont)
> fm2.1 <- lme(distance~age+Sex, data=dat, random=~1)
> 
> `dat' is an ordinary data.frame and thus random=~1 doesn't work. But 
> this works:
> 
> fm2.2 <- lme(distance~age+Sex, data=dat, random=~1|Subject)
> # you declare the grouping factor
> 
> I hope it helps.
> 
> Best,
> Dimitris

Thank you very much, now I see, why the Orthodont example works. There 
is however an important difference. In the example the random effects 
structure is not only ~1 but in reality ~1|Subject, which is inherited 
from the groupedData object. So

ranef(fm1)

yields 27 intercepts, but what I want is only one single intercept.

Thomas