Omitted Interactions in Negative Binomial Panel Regression

[Tom Menard]

Hello,

I ran into some trouble when introducing interaction effects in my model.
To quickly explain the variables:

• Patents = DV, count of patents in a given year
• Director = binary measure, indicating whether a director position was granted (1 if yes, 0 if not)
• Whollyowned = binary measure, indicating the organizational structure of a company (1 if a whollyowned-subsidiary, 0 if not)
• Direct = binary measure, also indicating the organizational structure of a company (1 if direct, 0 if not)

My goal is to test whether there are differences in the effect of director on patenting depending on the organizational structure of a company.

This is the shortened regression I ran:

xtset id year
xtnbreg patents L.director##L.whollyowned L.director##L.direct i.year

However, Stata does the following:

note: 1L.director#1L.direct omitted because of collinearity

Now I do not know if this an issue with my data, if my code is wrong or if my approach to test what I want is wrong in the first place.

I appreciate any help!

Thanks in advance.



Here is also a shortened dataex example:

* Example generated by -dataex-. To install: ssc install dataex
clear
input float(id year director no_director whollyowned direct) double patents
 1 2000 0 0 0 0  0
 1 2001 0 0 0 0  0
 1 2002 0 0 0 0  0
 1 2003 0 0 0 0  0
 1 2004 0 1 0 1  0
 1 2005 0 0 0 0  0
 1 2006 0 0 0 0  0
 1 2007 0 0 0 0  0
 1 2008 0 0 0 0  0
 1 2009 0 0 0 0  0
 1 2010 0 0 0 0  0
 1 2011 0 0 0 0  0
 1 2012 0 0 0 0  0
 2 2008 0 0 0 0  0
 2 2009 0 0 0 0  0
 2 2010 0 0 0 0  0
 2 2011 0 0 0 0  0
 2 2012 0 0 0 0  0
 3 2007 0 0 0 0  0
 3 2008 0 0 0 0  0
 3 2009 0 0 0 0  0
 3 2010 0 0 0 0  0
 3 2011 0 0 0 0  0
 3 2012 0 0 0 0  0
 4 2007 0 0 0 0  0
 4 2008 0 0 0 0  0
 4 2009 0 0 0 0  0
 4 2010 0 0 0 0  1
 4 2011 0 0 0 0  0
 4 2012 0 0 0 0  0
 5 2006 0 0 0 0  0
 5 2007 0 0 0 0  0
 5 2008 0 0 0 0  1
 5 2009 0 0 0 0  0
 5 2010 0 0 0 0  0
 5 2011 0 0 0 0  0
 5 2012 0 0 0 0  1
 6 2001 0 0 0 0  0
 6 2002 0 0 0 0  0
 6 2003 0 0 0 0  0
 6 2004 0 0 0 0  0
 6 2005 0 0 0 0  0
 6 2006 0 0 0 0  0
 6 2007 0 0 0 0  0
 6 2008 0 0 0 0  0
 6 2009 0 0 0 0  0
 6 2010 0 0 0 0  0
 6 2011 0 0 0 0  0
 6 2012 0 0 0 0  0
 7 2005 0 0 0 0  0
 7 2006 0 0 0 0  0
 7 2007 0 0 0 0  0
 7 2008 0 0 0 0  0
 7 2009 0 0 0 0  0
 7 2010 0 0 0 0  0
 7 2011 0 0 0 0  0
 7 2012 0 0 0 0  0
 8 2001 0 0 0 0  0
 8 2002 0 0 0 0  0
 8 2003 0 0 0 0  1
 8 2004 0 0 0 0  0
 8 2005 0 0 0 0  1
 8 2006 0 0 0 0  0
 8 2007 0 0 0 0  0
 8 2008 0 0 0 0  1
 8 2009 0 0 0 0  0
 8 2010 0 0 0 0  0
 8 2011 0 0 0 0  2
 8 2012 0 0 0 0  0
 9 2005 0 0 0 0  0
 9 2006 0 0 0 0  0
 9 2007 0 0 0 0  1
 9 2008 0 0 0 0  4
 9 2009 0 0 0 0  2
 9 2010 0 0 0 0  1
 9 2011 0 0 0 0  0
 9 2012 0 0 0 0  1
10 2004 0 0 0 0  0
10 2005 0 0 0 0  0
10 2006 0 0 0 0  0
10 2007 0 0 0 0  0
10 2008 0 0 0 0  0
10 2009 0 0 0 0  0
10 2010 0 0 0 0  0
10 2011 0 0 0 0  0
10 2012 0 0 0 0  0
11 2008 0 0 0 0  0
11 2009 0 0 0 0  0
11 2010 0 0 0 0  0
11 2011 0 0 0 0  0
11 2012 0 0 0 0  0
12 2010 0 0 0 0  0
12 2011 0 0 0 0  0
12 2012 0 0 0 0  0
13 2000 0 0 0 0  0
13 2001 0 0 0 0  0
13 2002 0 0 0 0  1
13 2003 0 0 0 0  0
13 2004 0 0 0 0  0
13 2005 0 0 0 0  0
13 2006 0 0 0 0  0
13 2007 0 0 0 0  0
13 2008 0 0 0 0  0
13 2009 0 0 0 0  0
13 2010 0 0 0 0  0
13 2011 0 0 0 0  0
13 2012 0 0 0 0  0
14 2002 0 0 0 0  0
14 2003 0 0 0 0  0
14 2004 0 0 0 0  0
14 2005 0 0 0 0  0
14 2006 0 0 0 0  0
14 2007 0 0 0 0  0
14 2008 0 0 0 0  0
14 2009 0 0 0 0  0
14 2010 0 0 0 0  0
14 2011 0 0 0 0  0
14 2012 0 0 0 0  0
15 2009 0 0 0 0  1
15 2010 1 0 1 0  0
15 2011 0 0 0 0  1
15 2012 0 0 0 0  0
16 2004 0 0 0 0  0
16 2005 0 0 0 0  6
16 2006 0 0 0 0  2
16 2007 0 0 0 0  6
16 2008 0 0 0 0  9
16 2009 0 0 0 0  5
16 2010 0 0 0 0 10
16 2011 1 0 0 1  3
16 2012 0 0 0 0 15
17 2002 0 0 0 0  0
17 2003 0 0 0 0  0
17 2004 0 1 0 1  0
17 2005 0 0 0 0  0
17 2006 0 0 0 0  0
17 2007 0 0 0 0  0
17 2008 0 0 0 0  0
17 2009 0 0 0 0  1
17 2010 0 0 0 0  1
17 2011 0 0 0 0  2
17 2012 0 0 0 0  0
18 2001 0 0 0 0  0
18 2002 0 0 0 0  0
18 2003 0 0 0 0  0
18 2004 0 0 0 0  0
18 2005 0 0 0 0  0
18 2006 0 0 0 0  0
18 2007 0 0 0 0  0
18 2008 0 0 0 0  0
18 2009 0 0 0 0  0
18 2010 0 0 0 0  0
18 2011 0 0 0 0  0
18 2012 0 0 0 0  0
19 2005 0 0 0 0  0
19 2006 0 1 1 0  0
19 2007 0 0 0 0  9
19 2008 0 0 0 0  1
19 2009 0 0 0 0  0
19 2010 0 1 1 0  1
19 2011 0 0 0 0  1
19 2012 0 0 0 0  5
20 2008 0 0 0 0  0
20 2009 0 0 0 0  0
20 2010 0 0 0 0  0
20 2011 0 0 0 0  3
20 2012 0 0 0 0  1
21 2002 0 0 0 0  0
21 2003 0 0 0 0  0
21 2004 0 0 0 0  1
21 2005 0 0 0 0  0
21 2006 0 0 0 0  0
21 2007 0 0 0 0  0
21 2008 0 0 0 0  0
21 2009 0 0 0 0  1
21 2010 0 0 0 0  1
21 2011 0 0 0 0  9
21 2012 0 0 0 0  2
22 2009 0 0 0 0  0
22 2010 0 0 0 0  0
22 2011 0 0 0 0  0
22 2012 0 0 0 0  0
23 2004 0 0 0 0  1
23 2005 1 0 1 0  1
23 2006 0 0 0 0  0
23 2007 1 0 1 0  0
23 2008 1 0 1 0  4
23 2009 0 0 0 0  1
23 2010 0 0 0 0  1
23 2011 0 0 0 0  0
23 2012 0 0 0 0  0
24 2009 0 0 0 0  0
24 2010 0 0 0 0  0
24 2011 0 0 0 0  0
24 2012 0 0 0 0  1
25 2000 0 0 0 0  0
25 2001 0 1 1 0  0
25 2002 0 0 0 0  2
25 2003 0 1 1 0  1
25 2004 0 1 1 0  1
25 2005 0 0 0 0  0
25 2006 0 0 0 0  0
25 2007 0 0 0 0  0
25 2008 0 0 0 0  0
25 2009 0 0 0 0  0
25 2010 0 0 0 0  0
25 2011 0 0 0 0  0
25 2012 0 0 0 0  0
26 2002 0 0 0 0  0
26 2003 0 0 0 0  0
26 2004 0 1 1 0  0
26 2005 0 0 0 0  0
26 2006 0 0 0 0  0
26 2007 0 0 0 0  0
26 2008 0 0 0 0  0
26 2009 0 0 0 0  0
26 2010 0 0 0 0  0
26 2011 0 0 0 0  0
26 2012 0 0 0 0  0
27 2006 0 0 0 0  0
27 2007 0 0 0 0  0
27 2008 0 0 0 0  0
27 2009 0 0 0 0  0
27 2010 0 0 0 0  0
27 2011 0 0 0 0  0
27 2012 0 0 0 0  0
28 2009 0 0 0 0  0
28 2010 1 0 0 1  0
28 2011 1 0 0 1  0
28 2012 0 0 0 0  0
29 2008 0 0 0 0  0
29 2009 0 0 0 0  0
29 2010 0 0 0 0  0
29 2011 0 0 0 0  0
29 2012 0 0 0 0  1
30 2004 0 0 0 0  0
30 2005 0 0 0 0  3
30 2006 0 0 0 0  1
30 2007 0 0 0 0  0
30 2008 0 0 0 0  1
30 2009 0 1 1 0  2
30 2010 0 0 0 0  4
30 2011 0 0 0 0  1
30 2012 0 0 0 0  5
31 2007 0 0 0 0  0
31 2008 0 0 0 0  0
31 2009 0 0 0 0  0
31 2010 0 0 0 0  0
31 2011 0 0 0 0  0
31 2012 0 0 0 0  0
32 2006 0 1 1 0  0
32 2007 0 1 1 0  0
32 2008 0 1 1 0  0
32 2009 0 0 0 0  0
32 2010 0 0 0 0  0
32 2011 0 0 0 0  1
32 2012 0 1 1 0  0
33 2007 0 0 0 0  0
33 2008 0 0 0 0  1
33 2009 1 0 1 0  1
33 2010 1 0 1 0  0
33 2011 0 0 0 0  1
33 2012 0 0 0 0  0
34 2003 0 0 0 0  0
34 2004 0 0 0 0  0
34 2005 0 0 0 0  0
34 2006 0 0 0 0  1
34 2007 0 0 0 0  1
34 2008 0 0 0 0  0
34 2009 0 0 0 0  0
34 2010 0 0 0 0  1
34 2011 0 0 0 0  0
34 2012 0 0 0 0  0
35 2002 0 0 0 0  1
35 2003 0 0 0 0  0
35 2004 0 0 0 0  0
35 2005 0 0 0 0  0
35 2006 0 0 0 0  0
35 2007 0 0 0 0  0
35 2008 0 0 0 0  0
35 2009 0 0 0 0  0
35 2010 0 0 0 0  0
35 2011 0 0 0 0  0
35 2012 0 0 0 0  0
36 2008 0 0 0 0  0
36 2009 0 0 0 0  0
36 2010 0 0 0 0  0
36 2011 0 0 0 0  0
36 2012 0 0 0 0  0
37 2002 0 0 0 0  0
37 2003 0 0 0 0  0
37 2004 0 0 0 0  2
37 2005 0 0 0 0  0
37 2006 0 0 0 0  0
37 2007 0 0 0 0  1
37 2008 0 0 0 0  1
37 2009 0 0 0 0  1
37 2010 0 0 0 0  1
37 2011 0 0 0 0  0
37 2012 0 0 0 0  0
end