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# st: Specify omitted group in regression

 From Simone Peart Boyce To Stata 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?

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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```