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Re: st: Correlation between 2 variables overtime- accounting for repeated measures


From   "JVerkuilen (Gmail)" <jvverkuilen@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Correlation between 2 variables overtime- accounting for repeated measures
Date   Sat, 16 Mar 2013 11:21:50 -0400

On Sat, Mar 16, 2013 at 10:13 AM, megan rossi <megan_rossi@msn.com> wrote:
> I was thinking of going with xtgee or xtmixed but wasn't quite sure if one would be more appropriate then the other? ( I know gee is more relaxed with equal variance and normality assumptions).
> With respect to the correlation option I didn't mention that the patients had a uninephrectomy following baseline so I think that disturbs the assumption associated with the ar1 option ie. the association with time is not as straight forward . Do you think given this the unstructured option would be best?
>
> Thanks I will look into SEM too....not sure how that would work, if you know of any websites with an example that would be much appreciated!

The Stata manual for SEM is pretty good and has numerous examples and
citations.

I'm not sure GEE would give you the correlation. xtmixed would let you
compute an intraclass correlation, which might be what you're looking
for. You don't have to have equal variances with xtmixed either.
There's quite a lot of flexibility to specify residual variance
structures.

My guess with the surgery is that you need to either model it (as a
fixed effect) since you do know when it happened, right?

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