st: ztnb model: why are some results omitted?

[Ekaterina Hertog <ekaterina.hertog@sociology.ox.ac.uk>]

Dear all,
I use Stata 12 and I am trying to analyse how individual relative income 
and whether the level of income was supported by official documentation 
or not affects the number of page views an person receives on a dating 
site. I am particularly interested in the interactions between the 
relative income variables and the variable for confirmed income.
I have no information about people who receive no page views so I use a 
zero-truncated negative binominal model.
The dependent variable is a count of the number of page views.
inclow25 = income is in the lowest quartile of incomes in the general 
population
inc50per = income in the 2nd lowest quartile
inc75per = income in the 2nd highest quartile
inchigh25 = income in the highest quartile of earners in the population
conf_inc = is a binary variable: 1 meaning that the income level is 
confirmed with appropriate documentation
I run the model separately for men and the code I use looks as follows:
ztnb totpagev inclow25##conf_inc inc50per##conf_inc inc75per##conf_inc 
inchigh25##conf_inc  if gender==1
In the resulting table some results are omitted and I am trying to 
understand why. The line for the interaction for the dummy variable for 
the lowest income quartile and the dummy variable for confirmed income 
is empty because there are no people in the lowest quartile who confirme 
their income levels. The situation is different for the highest income 
quartile. There are men who choose to confirm their income level and men 
who choose not to do so. Any pointers to why the results may be omitted 
in that case will be very much appreciated.
------------------------------------------------------------------------------------
          Totpagev        |      Coef.   Std. Err.      z    P>|z|     
[95% Conf. Interval]
-------------------+----------------------------------------------------------------
1.inclow25    |  -.2428269   .6096486    -0.40   0.690    -1.437716    
.9520623
1.conf_inc    |   .2210541   .0671414     3.29   0.001     .0894595    
.3526488
inclow25#conf_inc     |
              1 1          |          0  (empty)
1.inc50per     |  -.8697504   .0712843   -12.20   0.000    -1.009465   
-.7300357
inc50per#conf_inc     |
1 1          |  -.3711563   .2604532    -1.43   0.154    -.8816353    
.1393226
1.inc75per     |  -.5496872   .0503411   -10.92   0.000     -.648354   
-.4510204
 inc75per#conf_inc    |
1 1        |  -.0772988   .1831623    -0.42   0.673    -.4362904    
.2816928       1.inchigh25    |          0  (omitted)
inchigh25#conf_inc    |
1 1          |          0  (omitted)
             _cons         |   5.335535   .0257688   207.05   0.000     
5.285029    5.386041
-------------------+----------------------------------------------------------------
          /lnalpha         |   .1024199   .0261943                      
.0510801    .1537598
-------------------+----------------------------------------------------------------
             alpha         |   1.107849   .0290193                      
1.052407    1.166211
------------------------------------------------------------------------------------
Likelihood-ratio test of alpha=0:  chibar2(01) = 6.2e+05 Prob>=chibar2 = 
0.000
Ekaterina

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