Dear all,

I am currently analyzing how much similarity mother and father (spouses) show in their years of schooling, that is how high is the degree of assortative mating.

I have nine different birth cohorts in my data, each spanning four years. Thereby I want to use the opportunity to observe this behavior over time.

I first use a simple linear regression

Code:
reg years_schooling_G3_mother years_schooling_G3_father if (...) & cohort ==1
reg years_schooling_G3_mother years_schooling_G3_father if (...) & cohort ==2
reg years_schooling_G3_mother years_schooling_G3_father if (...) & cohort ==5
for all nine birth cohorts.

Then I estimate a second regression

Code:
reg years_schooling_G3_mother years_schooling_G3_father i.cohort if (...)
reg years_schooling_G3_mother years_schooling_G3_father cohort if (...)
However, I am quite surprised by the results. While the first model shows lower coefficients for higher cohorts, the second and third show (as far as I understood) increasing coefficients over cohorts.

Can anybody explain my misunderstanding? As all coefficients are significant, this cannot be the answer and I might have a logical error...


Thank you!



Code:
------------------------------------------------------------
                      (1)             (2)             (3)  
             ~g_G3_mother    ~g_G3_mother    ~g_G3_mother  
------------------------------------------------------------
~g_G3_father        0.491***        0.467***        0.459***
                  (18.89)         (19.88)         (21.70)  

_cons               5.108***        5.457***        5.911***
                  (19.69)         (22.87)         (25.85)  
------------------------------------------------------------
N                    2522            3061            3623  
------------------------------------------------------------
t statistics in parentheses
* p<0.05, ** p<0.01, *** p<0.001

Code:
-------------------------------------------------------------------------------------------
years_schooling_G3_mother |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
--------------------------+----------------------------------------------------------------
years_schooling_G3_father |   .4817556   .0055682    86.52   0.000     .4708416    .4926697
                          |
                   cohort |
                       2  |   .0991928   .0360512     2.75   0.006     .0285298    .1698558
                       3  |   .2796893   .0340529     8.21   0.000     .2129429    .3464356
                       4  |   .4186461   .0335497    12.48   0.000     .3528862    .4844059
                       5  |   .4601548   .0348339    13.21   0.000     .3918777    .5284318
                       6  |   .4614515   .0399617    11.55   0.000     .3831234    .5397795
                       7  |   .4049299   .0484335     8.36   0.000     .3099966    .4998631
                       8  |   .1815389   .0696623     2.61   0.009     .0449956    .3180822
                       9  |   -.000846    .155064    -0.01   0.996    -.3047831    .3030911
                          |
                    _cons |   5.203955   .0639659    81.36   0.000     5.078577    5.329333
-------------------------------------------------------------------------------------------


Code:
-------------------------------------------------------------------------------------------
years_schooling_G3_mother |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
--------------------------+----------------------------------------------------------------
years_schooling_G3_father |   .4873492   .0055614    87.63   0.000     .4764484      .49825
                   cohort |   .0705557   .0051374    13.73   0.000      .060486    .0806254
                    _cons |   5.177953   .0626206    82.69   0.000     5.055212    5.300694
-------------------------------------------------------------------------------------------