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st: logit, cluster() followed by test


From   "Alves, Bernadette" <b.alves@ic.ac.uk>
To   "'statalist@hsphsun2.harvard.edu.'" <statalist@hsphsun2.harvard.edu>
Subject   st: logit, cluster() followed by test
Date   Sat, 3 Aug 2002 13:31:03 +0100

I am analysing a dataset of 300,000 people in 90 hospitals.  My outcome
variable is elective caesarean and I'm looking at four independent
variables: age, gestation, deliveries per doctor and doctors per bed.  Below
I have reproduced a small sample of output.  As you can see, neither deldoc
nor docbed are significant predictors in the model but when I test if they
are jointly significant (using "test"), I find they have a p-value of
0.0175.  My question is should I include neither, either or both of them in
my final model?  They are clearly related to each other and if I include
either one of them they have p-values of less than 0.05.

Many thanks for your help
Bernadette





xi: logistic elec startage i.gestat deldoc docbed , robust cluster(provid)
i.gestat       _Igestat_4-6     (naturally coded; _Igestat_4 omitted)

Logit estimates                         Number of obs   =     193530
                                        Wald chi2(5)    =    1566.63
                                        Prob > chi2     =     0.0000
Log likelihood = -31761.251             Pseudo R2       =     0.1203

                  (standard errors adjusted for clustering on provid)
---------------------------------------------------------------------
             |             Robust
        elec | Odds Ratio Std. Err.  z  P>|z|   [95% Conf. Interval]
-------------+------------------------------------------------------
    startage | 1.081758 .0030363  28.00 0.000   1.075823  1.087725
  _Igestat_5 | .2074252 .0160341 -20.35 0.000   .1782638   .2413569
  _Igestat_6 | .0801571 .0061749 -32.76 0.000   .0689239   .093221
      deldoc | 1.000359 .0002938   1.22 0.221   .9997835  1.000935
      docbed | .9896782 .0057416  -1.79 0.074   .9784886  1.000996
---------------------------------------------------------------------
r; t=9.26 11:56:36

. test deldoc

 ( 1)  deldoc = 0.0

           chi2(  1) =    1.50
         Prob > chi2 =    0.2214

r; t=0.00 11:56:45

. test docbed

 ( 1)  docbed = 0.0

           chi2(  1) =    3.20
         Prob > chi2 =    0.0737

r; t=0.00 11:56:51

. test docbed deldoc

 ( 1)  docbed = 0.0
 ( 2)  deldoc = 0.0

           chi2(  2) =    8.10
         Prob > chi2 =    0.0175

r; t=0.00 11:56:56


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