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st: GEE and lag variables

From   "Alex Gamma" <[email protected]>
To   <[email protected]>
Subject   st: GEE and lag variables
Date   Wed, 13 Jul 2005 09:58:12 +0200

Dear Statasticians,

I have longitudinal data from an age cohort of 591 people at 6 time-points
over 20 years. I have two psychiatric diagnoses A and B, and I want to look
at the question of whether prior occurence of A predicts current A or B and
vice versa (i.e. whether prior B predicts current B or A). So I constructed
additional variables A_prior and B_prior coding for any prior occurence of
diagnosis A or B, and I ran the models

xtgee A A_prior B B_prior some_covariates, i(id) fam(bin) link(logit)
corr(exch) robust
xtgee B B_prior A A_prior some_covariates, i(id) fam(bin) link(logit)
corr(exch) robust

Two questions:

1) A sociologist colleague doubted that it is valid to include this kind of
"any prior occurence" variable, or indeed any lag-variable into the GEE
model, but I don't see any reason as to why not. But just to be sure I
checked with the experts before publishing these models: is he right?

2) If the A_prior and B_prior variables are admissible, is it correct to use
an exchangeable correlation structure or should I use an independent
structure? (this is what J.W.R. Twisk seems to recommend in "Applied
Longitudinal Data Analysis for Epidemiology", 2003, Cambridge University

Best regards,

Alex Gamma
University Hospital of Psychiatry
Research Department
Lenggstr. 31
8032 Zurich

email: [email protected]
phone: ++4144 384 2635
fax: ++4144 384 2446

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