Hi,

I am trying to look at the differences in income between African-Americans and White Americans. I am using panel data, and have run a fixed effects regression, which is the following:

Code:
xtreg linc i.highest_degree#bw i.bw#c.lhrswrk i.occup#bw ib2.urban_rural#bw i.marriage#bw i.bw#c.lwkswk_job i.census_region#bw, fe baselevel
However, I am also running into a problem with collinearity, although it is strange in that it seems almost systematic:
At the end of every interaction with a categorical variable, I have one value which is always omitted; that too always the very last category in the categorical variables. It doesn't happen for the continuous ones.

Code:
note: 1.educ#1.bw omitted because of collinearity
note: 10.occup#1.bw omitted because of collinearity
note: 2b.urban_rural#1.bw omitted because of collinearity
note: 5.marriage#1.bw omitted because of collinearity
note: 4.census_region#1.bw omitted because of collinearity
Code:
         marriage#bw |
Never married#White  |          0  (base)
Never married#Black  |  -.5309271   .0851924    -2.02   0.043    -.3394337   -.0054707
      Married#White  |   .8552037   .0204159     7.87   0.000     .1206852    .2007177
      Married#Black  |   .4691787   .0831313     0.69   0.489    -.1054266    .2204569
Separ, Divorc#White  |   .0947397   .0391489     4.68   0.000     .1065997    .2600673
Separ, Divorc#Black  |          0  (omitted)

                     |
     bw#c.lwkswk_job |
              White  |   .1013588    .004665    21.73   0.000     .0922152    .1105023
              Black  |   .1059799   .0079389    13.35   0.000     .0904193    .1215405
Is this a so-called 'technical problem', a problem with my formulation, or is it actually due to collinearity that STATA is finding in the data? Thank you very much.