# Why is my variable being omitted by stata?

I am carrying out a fixed effect regression. I have a dummy variable called female. The dependent variable docvis refers to hospital visits. I created an interaction term between hhkids and female called fekids. hhkids refers to whether or not a person has kids. I wanted to see whether women's hospital visits are more affected by having children than men's.

I have interpreted from the coefficient on fekids that women's hospital visits ARE more affected than men's. That women with children are 15.77% less likely to visit the hospital than men with children are. Is this interpretation correct.

I have included the variable female in my regression. I was told by someone that I do not need to include female. Why is this? Why is female omitted? I assume that this is due to the multicollinearity between female and fekids, however when I do an OLS regression this does not happen. Why is that?

  areg  docvis hhkids age agesq married working linc addon female fekid, absorb(id)
note: female omitted because of collinearity

Linear regression, absorbing indicators                Number of obs =    6209
F(  8,  5314) =    8.25
Prob > F      =  0.0000
R-squared     =  0.4187
Root MSE      =  4.5747

------------------------------------------------------------------------------
docvis |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
hhkids |   .7493504   .2887167     2.60   0.009     .1833471    1.315354
age |  -.2326124   .1003786    -2.32   0.021    -.4293957   -.0358292
agesq |   .0038731   .0010802     3.59   0.000     .0017553    .0059908
married |  -.0923826   .3659839    -0.25   0.801    -.8098612     .625096
working |  -.5702973   .2491114    -2.29   0.022    -1.058658   -.0819367
linc |   .0886328     .23889     0.37   0.711    -.3796897    .5569553
addon |   .3009833   .6369426     0.47   0.637    -.9476857    1.549652
female |  (omitted)
fekid |  -.1577091   .4279726    -0.37   0.713    -.9967111    .6812929
_cons |   5.793355   2.426897     2.39   0.017     1.035641    10.55107
-------------+----------------------------------------------------------------
id |      F(886, 5314) =      3.929   0.000         (887 categories)