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st: RE: Area under the curve trouble with aucdist


From   Joseph.Wagner2@med.va.gov
To   statalist@hsphsun2.harvard.edu
Subject   st: RE: Area under the curve trouble with aucdist
Date   Wed, 25 Sep 2002 06:55:50 -0500

After checking with describe, my data are all numeric though -time- is in
STATA date format which may or may not be a problem with aucdist.  (Please
see my response to Thomas Steichen's message)

Message: 16
   Date: Tue, 24 Sep 2002 19:20:08 +0100
   From: "Nick Cox" <n.j.cox@durham.ac.uk>
Subject: st: RE: Area under the curve trouble with aucdist

Joseph.Wagner2@med.va.gov
> 
> I need to calculate the area under the curve (viral load 
> over time) for
> about 1000 study participants using the trapezoidal rule.  
> I have used
> aucdist but I am not getting a result and am not sure if 
> I'm getting an
> error message.
> 
> id = patient id
> time =  time of viral load test
> conc = viral load value
> 
> aucdist   id   time   conc,   t  generate(auc)
> 
> (t specifies that the integral be computed using the simple 
> trapezoidal
> rule, and generate(auc) specifies to create a new variable 
> with the AUC (C
> max, T max, log conc 0 -inf.))
> 
> but all I'm getting is  (2000) in return which is supposed 
> to mean that I
> have no observations on which to perform the statistical operation.
> 
> I have 9500 observations with multiple observations per 
> patient, there are
> no missing data.  Any suggestions as to what I may be doing wrong?

Probably something is really a string variable 
which shouldn't be, itself possibly a legacy from some 
previous incarnation of the data within Microsoft software, 
or just that your -id- contains names. 

Take a look with -describe- and -destring- or -encode- 
any offender.

r(2000) strictly means "no numeric values
on which to do that". 

I have previously suggested to Stata Corp that
"no observations" is not the best wording for 
this error. 

Of course, I approve wholeheartedly of their 
reluctance to change longstanding syntax, 
which is a hangover from Stata 1.0, I guess, 
when there were no string variables. 

Nick 
n.j.cox@durham.ac.uk 

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