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

From: Giuseppe Pagnoni <gpagnon_at_emory.edu>
Date: Fri 21 Jul 2006 - 00:02:10 EST


Dear all,

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

Giuseppe Pagnoni
Dept. of Psychiatry
101 Woodruff Circle
Altanta, GA 30322
Tel: 404-712-9582
Fax: 404-727-3233

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