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st: xtgee and corr(ind) vs corr(exch)

Subject   st: xtgee and corr(ind) vs corr(exch)
Date   Sat, 02 Nov 2002 01:42:03 +1000

Dear Statalist,

Why is it that a coefficient can differ by up to 4 standard errors when an xtgee with corr(ind) is fitted compared to an xtgee with corr(exch) ?

I have about 20 clinics and 40 patients per clinic, with the one covarite varying at the patient level, and the id variable is the clinic. The intraclass correlation coefficient is about 0.4. My understanding is that the expectation is that the coefficient should not differ when a different correlation structure is fitted. The coefficient goes toward the null when the exchangeable structure is fitted.

Kind regards, Garry

Vet. Science, Univ of Melbourne


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