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st: Specify omitted group in regression
From
Simone Peart Boyce <[email protected]>
To
Stata <[email protected]>
Subject
st: Specify omitted group in regression
Date
Tue, 20 Nov 2012 14:52:13 -0800 (PST)
Hi,
I am running a regression with interacted factors and would like to specify the factors to use as the base. I do the following, but it seems only to work when the factor is not interacted.
fvset base 1 agencytype
xtgee breast i.agencytype bps i.agencytype#i.bps, family(binomial) link(logit) corr(exc) vce(robust)
The output as below omits agencytype=6 when interacted with BPS rather than agencytype=1. How do I get Stata to omit agencytype 1 instead?
Thanks in advance,
Simone
note: 2.agencytype#1.BPS identifies no observations in the sample
note: 6.agencytype#1.BPS omitted because of collinearity
Iteration 1: tolerance = .02508496
Iteration 2: tolerance = .00012234
Iteration 3: tolerance = 4.649e-07
GEE population-averaged model Number of obs = 16426
Group variable: patient_id Number of groups = 14253
Link: logit Obs per group: min = 1
Family: binomial avg = 1.2
Correlation: exchangeable max = 6
Wald chi2(10) = 220.51
Scale parameter: 1 Prob > chi2 = 0.0000
(Std. Err. adjusted for clustering on patient_id)
--------------------------------------------------------------------------------
| Robust
breast_compl~r | Coef. Std. Err. z P>|z| [95% Conf. Interval]
---------------+----------------------------------------------------------------
agencytype |
1 | 0 (base)
2 | -.3476051 .4017287 -0.87 0.387 -1.134979 .4397687
3 | .6622408 .1231947 5.38 0.000 .4207837 .9036979
4 | .303693 .3774096 0.80 0.421 -.4360163 1.043402
5 | .4469481 .1493272 2.99 0.003 .1542722 .7396241
6 | -.2703296 .1281538 -2.11 0.035 -.5215065 -.0191527
|
BPS | .2035705 .1517018 1.34 0.180 -.0937596 .5009006
|
agencytype#BPS |
1 1 | .6664529 .1794499 3.71 0.000 .3147376 1.018168
2 1 | 0 (empty)
3 1 | .77403 .2479991 3.12 0.002 .2879608 1.260099
4 1 | .8264168 .5002503 1.65 0.099 -.1540558 1.806889
5 1 | .1369255 .2521904 0.54 0.587 -.3573586 .6312096
6 1 | 0 (omitted)
|
_cons | 2.319875 .0603756 38.42 0.000 2.201541 2.438209
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