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st: ztnb model: why are some results omitted?


From   Ekaterina Hertog <ekaterina.hertog@sociology.ox.ac.uk>
To   statalist@hsphsun2.harvard.edu
Subject   st: ztnb model: why are some results omitted?
Date   Fri, 16 Mar 2012 22:14:24 +0000

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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