st: RE: RE: Reference group for categorical interactions

["Valle, Giuseppina" <gvalle@prc.utexas.edu>]

Thank you. That was a thought that crossed my mind. Do you know any way of reconciling this?

Thanks,
Pina
________________________________________
From: owner-statalist@hsphsun2.harvard.edu <owner-statalist@hsphsun2.harvard.edu> on behalf of Hussein, Mustafa (Mustafa Hussien) <mhussei4@uthsc.edu>
Sent: Wednesday, September 25, 2013 4:13 PM
To: statalist@hsphsun2.harvard.edu
Subject: st: RE: Reference group for categorical interactions

Hi Pina,

I wonder if that's because of the odds ratios/marginal effects associated with your un-interacted terms "maleoral" and "agesex". Margins takes into account all the ways a given term is expressed on the rhs of your model.

Hope that helps.
Mustafa
________________________________________
From: owner-statalist@hsphsun2.harvard.edu [owner-statalist@hsphsun2.harvard.edu] on behalf of Valle, Giuseppina [gvalle@prc.utexas.edu]
Sent: Wednesday, September 25, 2013 3:20 PM
To: statalist@hsphsun2.harvard.edu
Subject: st: Reference group for categorical interactions

Hello. I am trying to examine an interaction between two categorical predictors, each with 4 categories. For both of the variables:

1 = early
2 = normative
3 = late
4 = none

I want 2 or "normative" to be the referent for both variables, so I use the code:

fvset base 2 maleoral
fvset base 2 agesex

Then for the regression, I run:

svy, subpop(if gender==1): logistic conflict i.race ///
        agew4 i.famstw1 fameduc paedmis faminc incmiss adrel ///
        i.respeduc i.respinc sexforce std nonmar ///
        i.maleoral i.agesex i.maleoral#i.agesex

I get the following for the interaction term:

maleoral#agesex |
           1 1  |   2.626719   1.802462     1.41   0.162     .6756814     10.2114
           1 3  |          1  (empty)
           1 4  |   .6300918   1.185243    -0.25   0.806      .015239    26.05259
           3 1  |   2.467789   2.386861     0.93   0.352     .3640513    16.72836
           3 3  |    2.14585   .7743234     2.12   0.036     1.050786    4.382122
           3 4  |   2.751272   1.981607     1.41   0.162     .6616117      11.441
           4 1  |   1.601467   1.969695     0.38   0.702     .1404772    18.25703
           4 3  |   .5788523   .4207023    -0.75   0.453     .1374108    2.438454
           4 4  |   .3059179   .2770581    -1.31   0.193     .0509737     1.83596

I thought my reference group was respondents who were normative on "maleoral" AND normative on "agesex." But when I graph these, the results do not line up.

When I run the code:

margins maleoral#agesex, subpop(if gender==1) atmeans

Here is what I get:

---------------------------------------------------------------------------------
                |            Delta-method
                |     Margin   Std. Err.      z    P>|z|     [95% Conf. Interval]
----------------+----------------------------------------------------------------
maleoral#agesex |
           1 1  |   .2800785   .0805011     3.48   0.001     .1222993    .4378577
           1 2  |   .2389972   .1031905     2.32   0.021     .0367476    .4412468
           1 3  |          .  (not estimable)
           1 4  |   .0800896   .1295187     0.62   0.536    -.1737624    .3339417
           2 1  |   .1584989   .0410836     3.86   0.000     .0779765    .2390213
           2 2  |   .2854029   .0164211    17.38   0.000     .2532181    .3175876
           2 3  |   .1714103   .0296434     5.78   0.000     .1133102    .2295103
           2 4  |   .1494563   .0549283     2.72   0.007     .0417989    .2571137
           3 1  |   .1995144   .1381576     1.44   0.149    -.0712694    .4702983
           3 2  |   .1763853   .0405574     4.35   0.000     .0968943    .2558763
           3 3  |   .1922673   .0243652     7.89   0.000     .1445124    .2400222
           3 4  |   .2058666   .0774489     2.66   0.008     .0540695    .3576636
           4 1  |   .3372326   .2668201     1.26   0.206    -.1857253    .8601904
           4 2  |   .4025265    .108945     3.69   0.000     .1889982    .6160547
           4 3  |   .1680508   .0923209     1.82   0.069    -.0128949    .3489965
           4 4  |    .083139   .0436156     1.91   0.057     -.002346    .1686239

According to the odds ratios above, 3 and 3 (late and late) should results in a greater probability of my outcome compared to 2 and 2 (normative and normative), but according to the margins, 3 and 3 = 0.19 and 2 and 2 = 0.28. Am I confused about my reference group?

Any help would be greatly appreciated.

Thanks,
Pina Valle




*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/faqs/resources/statalist-faq/
*   http://www.ats.ucla.edu/stat/stata/

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/faqs/resources/statalist-faq/
*   http://www.ats.ucla.edu/stat/stata/

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/faqs/resources/statalist-faq/
*   http://www.ats.ucla.edu/stat/stata/