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  • GEE interaction effect - ommitted variables

    Hello statalist,

    I am researching the effect of chief digital officers on strategic change at a company by doing a GEE regression.

    One of the hypotheses that I test is: The association between CDO presence and strategic change is stronger if the CDO is part of a company’s executive committee.

    My panel dataset looks as follows. The dummy variable for executive committee is equal to one if the executive is part of the committee, zero if not, and empty if there is no chief digital officer in that firm-year.

    Code:
    * Example generated by -dataex-. For more info, type help dataex
    clear
    input long GVKEY double year float(cdo dummy_execComittee stratchange1)
    1045 2010 0 .  -2.601784
    1045 2011 0 . -2.0886009
    1045 2012 0 .  -2.873147
    1045 2013 0 . -1.2799462
    1045 2014 0 .  -.3453558
    1045 2015 0 . -1.2935414
    1045 2016 0 .  -1.763744
    1045 2017 0 .    -3.0115
    1045 2018 0 .  1.2935827
    1045 2019 0 .  1.2720878
    1075 2010 0 . -2.7677424
    1075 2011 0 .  -2.865253
    1075 2012 0 .  -2.969077
    1075 2013 0 . -2.9866655
    1075 2014 0 . -2.7382064
    1075 2015 0 . -2.9036155
    1075 2016 0 . -2.7266235
    1075 2017 0 .  -2.862787
    1075 2018 0 .  -2.978583
    1075 2019 0 .  -2.623896
    1078 2010 0 . -2.2792287
    1078 2011 0 .  -2.642496
    1078 2012 0 .  -2.967848
    1078 2013 0 . -1.9837472
    1078 2014 0 . -2.9220934
    1078 2015 0 .  -2.764499
    1078 2016 0 .  -2.390353
    1078 2017 0 . -2.5110555
    1078 2018 0 .  -2.864998
    1078 2019 0 .  -2.858804
    1161 2010 0 .  -.9779071
    1161 2011 0 .  -2.132206
    1161 2012 0 .  -1.481019
    1161 2013 0 . -2.2021053
    1161 2014 0 .  -.7882497
    1161 2015 0 . -.10412782
    1161 2016 0 .  -.8612247
    1161 2017 0 . -1.9829502
    1161 2018 0 . -2.1478033
    1161 2019 0 . -2.3022478
    1209 2010 0 . -2.9827795
    1209 2011 0 . -3.0681744
    1209 2012 0 .  -2.670916
    1209 2013 0 . -2.6834695
    1209 2014 0 .  -2.852429
    1209 2015 0 .  -2.866161
    1209 2016 0 . -2.8276665
    1209 2017 0 . -2.5355625
    1209 2018 0 .  -3.042375
    1209 2019 0 .  -2.790608
    1230 2010 0 . -2.5241604
    1230 2011 0 .  -2.799176
    1230 2012 0 .  -2.786882
    1230 2013 0 .  -2.795616
    1230 2014 0 .  -2.962554
    1230 2015 0 . -2.9254184
    1230 2016 0 .  -2.481045
    1230 2017 0 . -2.7610176
    1230 2018 0 . -2.9477084
    1230 2019 0 .  -2.851113
    1300 2010 0 .  -2.806876
    1300 2011 0 .  -2.764426
    1300 2012 0 .  -2.892504
    1300 2013 0 .  -2.981084
    1300 2014 0 .  -2.958081
    1300 2015 0 . -2.9666114
    1300 2016 0 .  -2.558535
    1300 2017 0 .   -3.11228
    1300 2018 0 .  -2.816524
    1300 2019 0 . -2.7739556
    1327 2010 0 .  -2.691804
    1327 2011 0 .   -2.95882
    1327 2012 0 . -2.5993896
    1327 2013 0 .  -2.898453
    1327 2014 0 .  -2.931638
    1327 2015 0 .  -2.480188
    1327 2016 0 .  -2.428869
    1327 2017 0 .  -2.938254
    1327 2018 0 .  -2.670571
    1327 2019 0 . -2.0389254
    1380 2010 0 . -2.8321004
    1380 2011 0 .  -2.858678
    1380 2012 0 . -2.8684785
    1380 2013 0 .  -2.714155
    1380 2014 0 .  -2.525053
    1380 2015 0 .  -2.486863
    1380 2016 0 .  -2.705614
    1380 2017 0 .  -2.571207
    1380 2018 0 .  -2.983559
    1380 2019 0 .  -2.853369
    1440 2018 1 0  -2.887975
    1440 2019 1 0  -2.719169
    1440 2010 0 .  -2.624753
    1440 2011 0 .  -2.717351
    1440 2012 0 .  -2.800929
    1440 2013 0 .  -2.824792
    1440 2014 0 .   -2.67811
    1440 2015 0 . -3.0011826
    1440 2016 0 .  -2.528976
    1440 2017 0 .  -3.033558
    end

    When I implement it as below it ommits the direct effect of the variable cdo and of the interaction term between cdo and execCommittee. It measures only the direct effect of the execCommittee dummy, which makes sense as the variable is empty for all firm-years without cdo and therefore the regression runs only with the observations that have no missing values.
    Code:
    xtgee stratchange1 L.cdo##L.dummy_execComittee, family(Gaussian) link(identity) corr(ar) vce(robust)
    I am unsure whether this is the right implementation to test the above hypothesis. When I implement it differently (putting a zero instead of a missing value in any case, except from execComittee = 1, meaning no missing values in that variable), then the interaction term is ommitted because of collinearity.

    I think the issue here is that there is no observation where cdo = 0 and execCommittee = 1. But I don't know how to test the hypothesis then.

    Thanks in advance for your help and advice.

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