st: Interpreting results from heckprob

["Florian Seliger" <florian.seliger@gmx.de>]

Dear Statalist,

I'm trying to estimate a heckprobit model with CIS data.
The dependent variable in the selection equation is whether a firm has introduced product innovations.
The dependent variable in the outcome equation is whether a firm (that has introduced product innovations) has introduced market novelties.

product innovation yes------->market novelty------>yes
                   no-------->-             ------>no

I'm using a set of control variables such as the log of the number of employees, R&D activities, the proportion of employees with higher education etc., the model variables I'm interested in, and different exclusion restrictions.

The problem is that coefficients of the log number of employees, R&D activities, and the proportion of employees become insignificant in the outcome equation in every specification I have tried out.
Running a simple probit model, all coefficients are highly significant.

My question is:
- How can I interpret this result?
- Do I use the wrong model?

In addition, I would like to ask you if you have an idea which variable (from the CIS survey or additional data) to use as instrument in the selection equation.

Thank you!

Best wishes,
Florian
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