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RE: st: ANCOVA repeated measures error


From   Steve <cheesehead.in.tx@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   RE: st: ANCOVA repeated measures error
Date   Mon, 9 Nov 2009 14:52:29 -0600

Thanks Philip.

I tried the command and the coefficients were approximately equal to
the regression coefficient I had obtained with Carlo's method.  It's
interesting to note that the std. errors with the xtmixed analysis
were of a magnitude 2.5x larger than the errors I obtained with the
regression model using clustering.  I'll read up on all these commands
for STATA and hopefully get a better feel for these.  Thanks again.

- Steve



-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Philip
Ender
Sent: Monday, November 09, 2009 2:35 PM
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: ANCOVA repeated measures error

Steve-

What you have is a mixed model with drug as the between subject and
age within subject.  Further, the design is unbalanced.
I recomend using -xtmixed- for your analysis.  The code would look
something like this in Stata 11:

xtmixed ser_conc i.drug c.age || id:

In Stata 10 and earlier the code looks like this:

xi: xtmixed ser_conc i.drug age || id:

-xtmixed- will deal with all of the repeatedness and unbalancedness
and automatically use the correct error for each term.

On 9nov2009 Steve <cheesehead.in.tx@gmail.com> wrote:

>...
>The data being analyzed includes some serum concentration values for
>an analyte (continuous variable) that was measured at multiple times
>during a 3 year period from study participants.  The number of
>measurements for each person ranges from 10-15.  The participants are
>also divided into two groups (drug+ and drug-).  Over time, as the
>person ages, the concentrations change.  We were trying to evaluate
>whether the rate of change... the slope... was different between the
>two groups.
>
>To do this, I thought that I could do an ANCOVA which includes a
>variable denoting repeated measures.  The syntax I used was:
>
>anova ser_conc drug age, continuous(age) repeated(id) partial
>
>where, id is a unique numeric id for each study subject.  The error I
>get states: "could not determine between-subject basic unit; use
>bseunit() option".  I've tried using the "bse" option but that doesn't
>help.  I've also included id in the model but that didn't help either.
>Obviously, I'm doing something wrong with the syntax but haven't been
>able to figure out the problem.
>...

Phil
-- 
Phil Ender
Statistical Consulting Group
UCLA  Academic Technology Services
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