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Re: st: correlation for longitudinal data


From   Ricardo Ovaldia <ovaldia@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: correlation for longitudinal data
Date   Mon, 20 Jul 2009 19:44:41 -0700 (PDT)

Austin Nichol wrote in reply to my question:

>Sure:
>corr y x

The problem is that I have non-independent observations. Longitudinal (panel) data. Multiple measurements per patient over time. Pearson's correlation assumes independence, therefore -corr- is not appropiate.

Thank you,
Ricardo.


Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK

Ricardo Ovaldia<ovaldia@yahoo.com> :
Sure:
corr y x

but I guess you have something else in mind--perhaps you could
clarify.  Maybe you want to allow mean and SD to change over time, and
standardize by time e.g.

webuse psidextract, clear
corr lwage ed
ren lwage y
ren ed x
egen my=mean(y), by(t)
egen sy=sd(y), by(t)
egen mx=mean(x), by(t)
egen sx=sd(x), by(t)
g zy=(y-my)/sy
g zx=(x-mx)/sx
reg zy zx, nocons
corr zy zx


On Mon, Jul 20, 2009 at 9:51 AM, Ricardo Ovaldia<ovaldia@yahoo.com> wrote:
>
> Dear all,
>
> Is there a way to estimate a correlation coefficient of two continuous measurements each collected over time on a sample of patients? Not sure if I can do this with -xtreg- for example.
>
> Thank you,
> Ricardo
>
>
> Ricardo Ovaldia, MS
> Statistician
> Oklahoma City, OK
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