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st: RE: Reference group for categorical interactions
From
"Hussein, Mustafa (Mustafa Hussien)" <[email protected]>
To
"[email protected]" <[email protected]>
Subject
st: RE: Reference group for categorical interactions
Date
Wed, 25 Sep 2013 21:13:38 +0000
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: [email protected] [[email protected]] on behalf of Valle, Giuseppina [[email protected]]
Sent: Wednesday, September 25, 2013 3:20 PM
To: [email protected]
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
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