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st: adjust after poisson


From   "Timothy J. Wade" <tjwade@cal.berkeley.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: adjust after poisson
Date   Fri, 08 Nov 2002 12:25:45 -0800

Sorry if this is a duplicate posting ( I tried to post from the yahoo website but was unsure of my success):

I'm trying to obtain adjusted rates by month after the following poisson regression:

xi: poisson count i.monthnum cycle, exposure(pyear)

when I type: . adjust cycle, by(monthnum) exp

I get the following error:
offset (ln(pyear)) not constant within by()

however, when I use type the same adjust command with the "nooffset" option. I get returned results that look approximately correct (see below).

My questions are: are the rates given after adjust with the nooffset option the correct adjusted rates? Also, why do I need to specify the nooffset option in adjust?
Thanks very much in advance!

(stata 7.0/Windows)

. adjust cycle, by(month) exp nooffset
----------------------
monthnum | exp(xb)
----------+-----------
1 | 2.38068
2 | 2.67684
3 | 2.18642
4 | 2.20107
5 | 2.09517
6 | 1.70584
7 | 2.01013
8 | 1.86005
9 | 1.64766
10 | 1.68153
11 | 2.37351
12 | 2.15014
----------------------
Key: exp(xb) = exp(xb)

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