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RE: st: comparing rates

From   "German Rodriguez" <>
To   <>
Subject   RE: st: comparing rates
Date   Mon, 8 Sep 2003 15:38:17 -0400


You could try a random-effects logit model with a doctor random effect:

xtlogit referred practice, re i(doctorid)

This will give you an estimate of the difference between the practices
in the usual log-odds scale.  It will also allow for intra-doctor
correlation, or the fact that some doctors are more likely to refer
patients than others. Stata will even give you a measure of intra-doctor
correlation (or see my article with Elo in the Stata journal 3(1) 32-46,
2003 for alternatives). 

The model does assume that the doctor's propensity to refer patients is
independent of the practice; you may think of this as both practices
drawing doctors from the same pool. But the model allows for a practice
effect on top of that, encouraging or discouraging referrals over and
above the doctor's own propensity.

An alternative is the conditional logit model suggested by Chris Rohlfs

clogit referred, group(doctorid)

This is akin to a fixed-effects model; it seems perfectly adequate to
me, but it doesn't give you an estimate of the practice effect because,
as Chris noted, doctors are nested within practices.

Germán Rodríguez

-----Original Message-----
[] On Behalf Of Ricardo
Sent: Monday, September 08, 2003 12:18 PM
Subject: Re: st: comparing rates

Dear all,

I have received only one reply to my post (please see
my original post below), so either of two things is
possible: (1) there is not a good answer or (2) my
question was no clear. So let me ask a slightly
different question.

Can I simply take the number of referrals at one
practice and divide by the number of patients seen at
that practice (referral rate for practice 1) and
compare it to the rate of  the other practice without
taking in account that some physicians within practice
may refer more than others? Or, should I compute the
referral rate for each physician, and then combine
these rates within practice by weighting them some
how?  If so how can I do this in Stata? what kind of
weights should I use?

Thank you,

--- Ricardo Ovaldia <> wrote:
> Dear all,
> We are interested in comparing the referral rates
> for
> two physician practices.   The fist group has 20
> physicians and the second group has 22 physicians.
> Each physician saw a variable number of similar
> patients some saw as few as 4 and others as many as
> 43. The physician either referred the patient to a
> specialist or did not. 
> If I compute the referral rate for each physician
> group and compare them, I concerned because the
> observations are not independent. I suspect that
> some
> doctors are more likely to refer patients than
> other.
> I though that I could use &#33192;logit- to group by
> doctor&#30196; ID:
> . clogit referred practice, group(doctorid)
> This did not work because the &#26420;ractice-
> does
> not vary within doctor ID, so it is dropped from the
> model (omitted due to no within-group variance).
> Any suggestions will be greatly appreciated.
> Thank you,
> Ricardo.
> =====
> Ricardo Ovaldia, MS
> Statistician 
> Oklahoma City, OK
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