-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 <[email protected]> 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
[email protected]
18 Cantine's Island
Saugerties NY 12477
USA
845-246-0774
On Mon, Nov 9, 2009 at 12:49 PM, Steve <[email protected]> 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
[email protected]
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/
