This is an interesting question that I have run into as well. It seems as
though you would need to account for the within id and within treatment
correlations. In an effort to spark a discussion on this, I have considered
the use of multilevel modeling which is possible with the gllamm command.
There is an article in the Stata Journal vol(2) Number(1) 2002 by S.
Rabe-Hesketh, et. al. Hope this helps.
Greg
-----Original Message-----
From: VIVIAND Xavier [mailto:[email protected]]
Sent: Friday, January 10, 2003 6:51 AM
To: '[email protected]'
Subject: st: RE: RE : GEE
> ----------
> De : [email protected]
> Envoy� : jeudi 9 janvier 2003 18:13
> � : [email protected]
> Objet : RE: GEE
>
> Xavier:
>
> Why would you want an autoregressive time-series correlation structure
> for this data? It appears to me that the exchangeable would be the one
> desired. The latter is the one that you would nornally want for
> longitudinal data, unless it is over a great many time frames.
>
> Joe Hilbe
>
>
>
Dear Joe,
Thank you for your rapid answer. I wanted to use autoregressive time-series
correlation structure after reading Davis (Statistical methods for the
analysis of repeated measurements, Springer, 2002): "Although assumption of
constant correlation between any 2 repeated measurements may not be
justified in a longitudinal study, it is often reasonable in situations in
which the repeated measurements are not obtained over time. The AR-1 model
is a natural one to consider when measurements are taken repeatedly over
time".
Finally, I will use the exchangeable correlation structure for experiment.
However I would be pleased to have my initial question answered: is it
possible using xtgee to take simultaneously into account two factors (for
example time and patients as in my data) ? Sincerely yours,
Xavier Viviand
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