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st: Plotting interactions


From   Amal Khanolkar <[email protected]>
To   "[email protected]" <[email protected]>
Subject   st: Plotting interactions
Date   Mon, 30 Sep 2013 08:41:25 +0000

Hi All,

I'm trying to plot interactions post regression using the following syntax:

The model:

. regress bwtgestage_sd i.ethnicity_bi2#i.smoke2 sex ib2.magecat i.parity ib2.education i.famsit_new ib2.MBMI5 gestage_wk if multibirth==1, vce(robust)

Linear regression                                      Number of obs = 1144571
                                                       F( 22,1144548) = 4543.46
                                                       Prob > F      =  0.0000
                                                       R-squared     =  0.0820
                                                       Root MSE      =  .94207

--------------------------------------------------------------------------------------
                     |               Robust
       bwtgestage_sd |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
---------------------+----------------------------------------------------------------
ethnicity_bi2#smoke2 |
                1 3  |  -.3777867   .0023854  -158.37   0.000    -.3824621   -.3731114
                2 2  |  -.1160349   .0063877   -18.17   0.000    -.1285545   -.1035153
                2 3  |  -.4195231   .0107743   -38.94   0.000    -.4406403   -.3984059
                3 2  |  -.4354954   .0044767   -97.28   0.000    -.4442696   -.4267213
                3 3  |   -.577776   .0153539   -37.63   0.000    -.6078691    -.547683
                     |
                 sex |   .0127302   .0017618     7.23   0.000     .0092772    .0161832
                     |
             magecat |
                  1  |   .0813647   .0062532    13.01   0.000     .0691086    .0936207
                  3  |  -.0483047   .0024836   -19.45   0.000    -.0531725   -.0434369
                  4  |  -.0756843   .0028012   -27.02   0.000    -.0811745    -.070194
                  5  |  -.1036756   .0037371   -27.74   0.000    -.1110003    -.096351
                  6  |  -.1385867   .0075963   -18.24   0.000    -.1534752   -.1236982
                     |
              parity |
                  2  |    .308966   .0020534   150.47   0.000     .3049415    .3129905
                  3  |   .4244317   .0026556   159.83   0.000     .4192269    .4296365
                     |
           education |
                  1  |   -.044489   .0029772   -14.94   0.000    -.0503241   -.0386539
                  3  |   .0334391   .0024809    13.48   0.000     .0285766    .0383016
                  4  |   .0482377   .0025474    18.94   0.000     .0432449    .0532304
                     |
          famsit_new |
                  3  |  -.0321022   .0055319    -5.80   0.000    -.0429445   -.0212598
                  4  |   -.023603   .0077085    -3.06   0.002    -.0387113   -.0084947
                     |
               MBMI5 |
                  1  |  -.2915349   .0039634   -73.56   0.000    -.2993029   -.2837669
                  3  |    .249358   .0023809   104.73   0.000     .2446914    .2540245
                  4  |   .3691747   .0042171    87.54   0.000     .3609092    .3774401
                     |
      gestage_wktemp |  -.0000848   .0005237    -0.16   0.871    -.0011111    .0009416
               _cons |  -.0348456   .0209644    -1.66   0.096    -.0759352     .006244


I then use the following :

qui foreach x of var magecat {
	sum `x', d
	replace `x' = r(p50)
	}
predict p
predict se, stdp
tw (line p ethnicity_bi2 if smoke2==2, sort) (line p ethnicity_bi2 if smoke2==3, sort) 


I would like to know if the line above 'qui foreach x of var magecat' actually does indicate all categories of all variables magecat onwards as specified in the model including the continuous variable gestaga_wk?

I don't seem to get a graph I was expecting  - or at least I can't make sense of it. Have I specified the graph correctly or is there a better way plot interactions from a regression model?

Thanks!
Amal 



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