[R] Corstr in the Gee (Generalized Estimation Equation) arguments?

[Jinn-Yuh Guh]

Dear all:
Y=a+bX1+cX2
In the Gee (Generalized Estimation Equation) arguments:
The arument Corstr has sveral choices:
            "independence" "fixed" "stat_M_dep" "non_stat_M_dep"
             "exchangeable" "AR-M" "unstructured" 
What does each term mean?
How do I choose among them?
How do I know the correlation structure of my repeated measures data?
Can non-normally distributed X1, X2 be used?
Thank you very much.

Sincerely,
Jinn-Yuh Guh
Kaohsiung Medical Colege
jyuh at mail.nsysu.edu.tw
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