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st: ztnb model: why are some results omitted?
From
Ekaterina Hertog <[email protected]>
To
[email protected]
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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