[R] (robust) mixed-effects model with covariate

[Giuseppe Pagnoni]

Dear all,

First of all I apologize if you received this twice: I was checking the 
archive and I noticed that the text was scrubbed from the message, 
probably due to some setting in my e-mail program.


I am unsure about how to specify a model in R and I thought of asking 
some advice to the list. I have two groups ("Group"= A, B) of subjects, 
with each subject undertaking a test before and after a certain 
treatment ("Time"= pre, post). Additionally, I want to enter
the age of the subject as a covariate (the performance on the test is 
affected by age), and I also want to allow different slopes for the 
effect of age in the two groups of subjects (age might affect the 
performance of the two groups differentially).

Is the right model to use something like the following?

aov (y ~ Group*Time + Group*Age + Error(Subj/Group), data=df1 )

(If I enter that command, within summary, I get the following:
Error() model is singular in: aov(y ~ Group * Time + Group * Age +
Error(Subj/Group), data = df1))


As a second question: I have an outlier in one of the two groups. The 
outlier is not due to a measurement error but simply to the performance 
of the subject (possibly related to his medical history, but I have no 
way to determine that with certainty). This subject is
signaled to be an outlier within its group: averaging the pre and post 
values for the performance of the subjects in his group, the Grubbs test 
yields a probability of 0.002 for the subject to be an outlier (the 
subject is marked as a significant outlier also if I
perform the test separately on the pre and the post data).

If I remove this subject from its group, I get significant effects of 
Group and Group X Age (not using the R formula above, but another stat 
software), but if I leave the subject in those effects disappear. Since 
I understand that removing outliers is always worrysome, I would like to 
know if it is possible in R to estimate a model similar to that outlined 
above but in a resistant/robust fashion, and what would be the actual 
syntax to do that. I will very much appreciate any help or suggestion 
about this.

thanks in advance and best regards

giuseppe

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Giuseppe Pagnoni
Psychiatry and Behavioral Sciences
Emory University School of Medicine
101 Woodruff Circle, Suite 4000
Atlanta, GA, 30322
tel: 404.712.8431
fax: 404.727.3233