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Re: st: ANCOVA repeated measures error


From   sjsamuels@gmail.com
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: ANCOVA repeated measures error
Date   Tue, 10 Nov 2009 12:26:17 -0500

-anova= (and ANCOVA) are the wrong approaches. In -anova-, the
repeated() option takes a categorical variable that is in the model.
You have a longitudinal data problem, and should consult that
literature.  See, for example, Diggle, P.J., Liang, K.Y. and Zeger,
S.L. Analysis of Longitudinal Data, Oxford Press, 1994 (there is a
more recent edition, I believe) and Applied Longitudinal Data
Analysis: Modeling Change and Event Occurrence by Judith D. Singer and
John B. Willett, Oxford.

Check out Stata's longitudinal manual and -xt- commands, particularly
-xtgls- and -xtreg-. Stata examples for the Singer-Willet book can be
found at the UCLA web site
http://www.ats.ucla.edu/stat/stata/examples/alda/default.htm

-Steve

On Mon, Nov 9, 2009 at 12:49 PM, Steve <cheesehead.in.tx@gmail.com> wrote:
> I recently moved to STATA from another statistical software and was
> trying to analyze some data using ANCOVA, but I keep getting an error
> message.
>
> 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.
>
> So what am I doing wrong there? Is the model totally wrong the way I
> have written it? I tried searching for help on this but didn't get
> far. Would appreciate any help I can get. Thanks a heap in advance.


Steven Samuels
sjsamuels@gmail.com
18 Cantine's Island
Saugerties NY 12477
USA
845-246-0774


On Mon, Nov 9, 2009 at 12:49 PM, Steve <cheesehead.in.tx@gmail.com> wrote:
> Hi all,
>
> I recently moved to STATA from another statistical software and was
> trying to analyze some data using ANCOVA, but I keep getting an error
> message.
>
> 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.
>
> So what am I doing wrong there?  Is the model totally wrong the way I
> have written it?  I tried searching for help on this but didn't get
> far.  Would appreciate any help I can get.  Thanks a heap in advance.
>
> - Steve
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
>



-- 
Steven Samuels
sjsamuels@gmail.com
18 Cantine's Island
Saugerties NY 12477
USA
845-246-0774

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/



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