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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 14:45:57 -0600

Thanks Dave,

I saw your reply only after posting my previous reply to Carlo.  As
for missing data, individuals did not have serum collected and
measured at the same frequency and interval.  The number of repeated
measures for an individual ranges from 10-15 over the study period
(some patients couldn't come in to clinics, etc).  Would this create
an issue in the model?  Can't it handle the unequal distribution of
repeated measures?

I did retry the model including a time variable (in weeks since first
sample) in the model and also as the repeated measure variable, but
still kept getting the bse error.

- Steve

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of David Airey
Sent: Monday, November 09, 2009 2:31 PM
To: Statalist
Subject: re: st: ANCOVA repeated measures error

.

Another problem is that your subject id is not the repeated measure
variable. It's time of measurement, which is not in your model.
Perhaps another problem is that you have missing data and you are not
clear that even for the dates, serum was measured, you don't say they
are the same across subjects.

-Dave


> 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
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