Hey,

I have the following stylized example.

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input float(student_performance student_female evaluator_female dyad_stud_eval) long(evaluator_num student_num)
12 1 1 2 2 1
13 1 1 2 2 1
14 1 1 4 2 2
10 1 1 4 2 2
 8 1 0 3 1 2
 9 0 0 5 1 3
 2 0 1 6 2 3
12 0 0 7 1 4
 9 0 0 7 1 4
 9 1 0 1 1 1
10 1 1 4 2 2
11 1 1 2 2 1
end
label values evaluator_num evaluator_num
label def evaluator_num 1 "X", modify
label def evaluator_num 2 "Y", modify
label values student_num student_num
label def student_num 1 "A", modify
label def student_num 2 "B", modify
label def student_num 3 "C", modify
label def student_num 4 "D", modify
where dyad_stud_eval is simply "egen dyad_stud_eval = group(student_num evaluator_num)

I want to interact the gender of the student with the gender of the evaluator and see if there is any moderation going on that effects the students performance.
Now I am thinking about the fixed-effect level and I would like to run both, student and evaluator fe since evaluators might differ in students evaluation besides gender (same for students).
Now I have several options to run the regressions:
Code:
reg student_performance student_female##evaluator_female
what yields

Code:
      Source |       SS           df       MS      Number of obs   =        12
-------------+----------------------------------   F(3, 8)         =     11.44
       Model |  85.0833333         3  28.3611111   Prob > F        =    0.0029
    Residual |  19.8333333         8  2.47916667   R-squared       =    0.8110
-------------+----------------------------------   Adj R-squared   =    0.7401
       Total |  104.916667        11  9.53787879   Root MSE        =    1.5745

-------------------------------------------------------------------------------------------------
            student_performance |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
--------------------------------+----------------------------------------------------------------
               1.student_female |       -1.5   1.437349    -1.04   0.327    -4.814533    1.814533
             1.evaluator_female |         -8   1.818119    -4.40   0.002    -12.19259   -3.807411
                                |
student_female#evaluator_female |
                           1 1  |   11.16667   2.226732     5.01   0.001     6.031815    16.30152
                                |
                          _cons |         10   .9090593    11.00   0.000     7.903705    12.09629
-------------------------------------------------------------------------------------------------
when using
Code:
areg student_performance student_female##evaluator_female, absorb(dyad_stud_eval)
obviously everything is omitted because the is no variation between the gender of the evaluator and the student.


Code:
Linear regression, absorbing indicators         Number of obs     =         12
Absorbed variable: dyad_stud_eval               No. of categories =          7
                                                F(   0,      5)   =          .
                                                Prob > F          =          .
                                                R-squared         =     0.8364
                                                Adj R-squared     =     0.6400
                                                Root MSE          =     1.8529

-------------------------------------------------------------------------------------------------
            student_performance |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
--------------------------------+----------------------------------------------------------------
               1.student_female |          0  (omitted)
             1.evaluator_female |          0  (omitted)
                                |
student_female#evaluator_female |
                           1 1  |          0  (omitted)
                                |
                          _cons |   9.916667   .5348936    18.54   0.000     8.541679    11.29165
-------------------------------------------------------------------------------------------------
F test of absorbed indicators: F(6, 5) = 4.260                Prob > F = 0.067
still I can estimate the "same" regression using
Code:
reg student_performance student_female##evaluator_female i.student_num i.evaluator_num
and get
Code:
      Source |       SS           df       MS      Number of obs   =        12
-------------+----------------------------------   F(5, 6)         =      6.12
       Model |  87.7083333         5  17.5416667   Prob > F        =    0.0238
    Residual |  17.2083333         6  2.86805556   R-squared       =    0.8360
-------------+----------------------------------   Adj R-squared   =    0.6993
       Total |  104.916667        11  9.53787879   Root MSE        =    1.6935

-------------------------------------------------------------------------------------------------
            student_performance |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
--------------------------------+----------------------------------------------------------------
               1.student_female |     -1.625   1.796263    -0.90   0.401    -6.020298    2.770298
             1.evaluator_female |         -7   2.395018    -2.92   0.027     -12.8604   -1.139602
                                |
student_female#evaluator_female |
                           1 1  |   10.16667   2.765529     3.68   0.010     3.399662    16.93367
                                |
                    student_num |
                             B  |       -.75   1.197509    -0.63   0.554    -3.680199    2.180199
                             C  |       -1.5   2.074146    -0.72   0.497    -6.575253    3.575253
                             D  |          0  (omitted)
                                |
                  evaluator_num |
                             Y  |          0  (omitted)
                          _cons |       10.5   1.197509     8.77   0.000     7.569801     13.4302
-------------------------------------------------------------------------------------------------
The question is: Is estimating a student and evaluator fixed effect still correct since there is no variation in the gender anymore?

Sorry for the long text!
Bests,
Julian