st: Correcting for self selection

[erik.aadland@bi.no]

Dear statalist. 

Based on an unbalanced panel data set of organizations competing with projects in a monthly competition, I try to model the likelihood of an organization winning (binary outcome) given explanatory variables. However, the organizations with the highest scores on the main explanatory variables (orgs a) participate more often in the contest than organizations with lower scores (orgs b).

Since (orgs a) participate often, they enter many projects that lose and some projects that win. As a consequence, even though (orgs a) win the most in the contest overall, the models produce negative coefficients for these (orgs a) organizations.

Any suggestions for procedures in stata that I can apply to correct for this bias?

Thanks,

Erik.
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