Re: st: ztnb model: why are some results omitted?

[David Hoaglin <dchoaglin@gmail.com>]

Dear Ekaterina,

How are you interpreting the constant term in the model?  Because the
four income categories are exhaustive, they need only three parameters
in addition to the constant term.  It seems likely that ztnb is
treating the highest quartile of income as the reference category.

I suggest that you merge the four income variables into a single
categorical variable (which you might name inccat) and then use
inccat##conf_inc as the list of independent variables.

Regards,

David Hoaglin

On Fri, Mar 16, 2012 at 6:14 PM, Ekaterina Hertog
<ekaterina.hertog@sociology.ox.ac.uk> wrote:
> 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
>

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