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st: -by- with -binreg-


From   Lee Sieswerda <Lee.Sieswerda@tbdhu.com>
To   "Statalist (E-mail)" <statalist@hsphsun2.harvard.edu>
Subject   st: -by- with -binreg-
Date   Wed, 4 Dec 2002 15:31:04 -0500

Dear Statalisters:
For some reason I cannot get -by- to work with -binreg-. I can get around
the problem using -if-, but according to the manual I should be able to use
-by-. Here is my log:

. table pre_post sex, c(mean q7cat)

------------------------------
          |        sex        
 pre_post |        F         M
----------+-------------------
      pre | .3238095  .4953271
     post |       .6  .6296296
------------------------------

* Both sexes together
. binreg q7cat pre_post, rd cluster(id)

Iteration 1 : deviance =  570.9529
Iteration 2 : deviance =  570.9529

Residual df  =       423                                No. of obs =
425
Pearson X2   =       425                                Deviance   =
570.9529
Dispersion   =  1.004728                                Dispersion =
1.349771

Bernoulli distribution, identity link
Risk difference coefficients
                               (standard errors adjusted for clustering on
id)
----------------------------------------------------------------------------
--
             |             Semi-Robust
       q7cat |      Coef.   Std. Err.      z    P>|z|     [95% Conf.
Interval]
-------------+--------------------------------------------------------------
--
    pre_post |   .2046461   .0391639     5.23   0.000     .1278862
.281406
       _cons |   .2057312   .0651479     3.16   0.002     .0780438
.3334187
----------------------------------------------------------------------------
--

* Try to get separate estimates by sex

. bysort sex: binreg q7cat pre_post, rd cluster(id)

____________________________________________________________________________
___
-> sex = F

Iteration 1 : deviance =  570.9529
Iteration 2 : deviance =  570.9529

Residual df  =       423                                No. of obs =
425
Pearson X2   =       425                                Deviance   =
570.9529
Dispersion   =  1.004728                                Dispersion =
1.349771

Bernoulli distribution, identity link
Risk difference coefficients
                               (standard errors adjusted for clustering on
id)
----------------------------------------------------------------------------
--
             |             Semi-Robust
       q7cat |      Coef.   Std. Err.      z    P>|z|     [95% Conf.
Interval]
-------------+--------------------------------------------------------------
--
    pre_post |   .2046461   .0391639     5.23   0.000     .1278862
.281406
       _cons |   .2057312   .0651479     3.16   0.002     .0780438
.3334187
----------------------------------------------------------------------------
--

____________________________________________________________________________
___
-> sex = M

Iteration 1 : deviance =  570.9529
Iteration 2 : deviance =  570.9529

Residual df  =       423                                No. of obs =
425
Pearson X2   =       425                                Deviance   =
570.9529
Dispersion   =  1.004728                                Dispersion =
1.349771

Bernoulli distribution, identity link
Risk difference coefficients
                               (standard errors adjusted for clustering on
id)
----------------------------------------------------------------------------
--
             |             Semi-Robust
       q7cat |      Coef.   Std. Err.      z    P>|z|     [95% Conf.
Interval]
-------------+--------------------------------------------------------------
--
    pre_post |   .2046461   .0391639     5.23   0.000     .1278862
.281406
       _cons |   .2057312   .0651479     3.16   0.002     .0780438
.3334187
----------------------------------------------------------------------------
--

* Since that gave the same results as if I hadn't used
* -by- at all, try -if- instead

. binreg q7cat pre_post if sex=="M", rd cluster(id)

Iteration 1 : deviance =  290.7011
Iteration 2 : deviance =  290.7011

Residual df  =       213                                No. of obs =
215
Pearson X2   =       215                                Deviance   =
290.7011
Dispersion   =   1.00939                                Dispersion =
1.364794

Bernoulli distribution, identity link
Risk difference coefficients
                               (standard errors adjusted for clustering on
id)
----------------------------------------------------------------------------
--
             |             Semi-Robust
       q7cat |      Coef.   Std. Err.      z    P>|z|     [95% Conf.
Interval]
-------------+--------------------------------------------------------------
--
    pre_post |   .1343025   .0536422     2.50   0.012     .0291657
.2394394
       _cons |   .3610246   .0910578     3.96   0.000     .1825546
.5394945
----------------------------------------------------------------------------
--

. binreg q7cat pre_post if sex=="F", rd cluster(id)

Iteration 1 : deviance =  273.5711
Iteration 2 : deviance =  273.5711

Residual df  =       208                                No. of obs =
210
Pearson X2   =       210                                Deviance   =
273.5711
Dispersion   =  1.009615                                Dispersion =
1.315246

Bernoulli distribution, identity link
Risk difference coefficients
                               (standard errors adjusted for clustering on
id)
----------------------------------------------------------------------------
--
             |             Semi-Robust
       q7cat |      Coef.   Std. Err.      z    P>|z|     [95% Conf.
Interval]
-------------+--------------------------------------------------------------
--
    pre_post |   .2761905   .0566061     4.88   0.000     .1652445
.3871365
       _cons |    .047619   .0911676     0.52   0.601    -.1310661
.2263042
----------------------------------------------------------------------------
--

* Suppose I use -regress- instead of -binreg-. Then can I use -by-?
. by sex: regress q7cat pre_post, cluster(id)

____________________________________________________________________________
___
-> sex = F

Regression with robust standard errors                 Number of obs =
210
                                                       F(  1,   104) =
23.69
                                                       Prob > F      =
0.0000
                                                       R-squared     =
0.0767
Number of clusters (id) = 105                          Root MSE      =
.48134

----------------------------------------------------------------------------
--
             |               Robust
       q7cat |      Coef.   Std. Err.      t    P>|t|     [95% Conf.
Interval]
-------------+--------------------------------------------------------------
--
    pre_post |   .2761905   .0567421     4.87   0.000     .1636688
.3887121
       _cons |    .047619   .0913865     0.52   0.603    -.1336038
.2288419
----------------------------------------------------------------------------
--

____________________________________________________________________________
___
-> sex = M

Regression with robust standard errors                 Number of obs =
215
                                                       F(  1,   108) =
6.24
                                                       Prob > F      =
0.0140
                                                       R-squared     =
0.0183
Number of clusters (id) = 109                          Root MSE      =
.49378

----------------------------------------------------------------------------
--
             |               Robust
       q7cat |      Coef.   Std. Err.      t    P>|t|     [95% Conf.
Interval]
-------------+--------------------------------------------------------------
--
    pre_post |   .1343025    .053768     2.50   0.014      .027725
.2408801
       _cons |   .3610246   .0912713     3.96   0.000     .1801091
.5419401
----------------------------------------------------------------------------
--

So in this case -regress- works with -by-, but -binreg- does not. I also
have sex as a numeric variable, but that doesn't alleviate the non-working
-by- (NPI). Any ideas?

Regards,
Lee

Lee Sieswerda, Epidemiologist
Thunder Bay District Health Unit
999 Balmoral Street
Thunder Bay, Ontario
Canada  P7B 6E7
Tel: +1 (807) 625-5957
Fax: +1 (807) 623-2369
Lee.Sieswerda@tbdhu.com
www.tbdhu.com

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