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: [email protected]
[mailto:[email protected]] On Behalf Of Philip
Ender
Sent: Monday, November 09, 2009 2:35 PM
To: [email protected]
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 <[email protected]> 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
*
* 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/
*
* 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/
