lme: error message with random=~1

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